I've sat in many 2011-2014 Fiestas and several 2012-2014 Focuses. I can tell you the interior quality is just about equal IMO. The interior quality doesn't get any better on the ST model of either other than maybe a color change or two. They use the same interior pieces they use on the $15K regular Fiestas as they do on the Fiesta ST. The Fiesta is inherently an econo-box so asking for super quality interior is not gonna happen.
That being said, your perception of quality is really what it all comes down to. I currently drive a 2003 Ford ZX2, which was Ford's cheapest car is 2003. It's got cheap interior all over that vibrates and buzzes every now and then with a nice air leak when going highway speed by the driver's door mirror. I don't really mind it. As I said I've driven multiple Fiestas, and 5 Fiesta STs on top of that. I can tell you that every one of the Fiestas I drive had a much better interior than my ZX2. I'm totally and completely happy with that. When I get a Fiesta and the interior starts making noises, oh well. I may track down the noise, but unless it's really loud then I probably won't care. If you're coming from luxury quality interiors down to a Fiesta quality interior, you will notice a difference I'm sure. Having never driven a luxury car myself, I'm plenty content with it. Another thing people go to is road noise, which I also laugh about because my ZX2 is about as loud as they get. The Fiesta ST isn't silent, but if you want silent, the Fiesta is not the car for you. You can add sound deadener, but that adds weight (not good for the whole power to weight ratio thing).
While I haven't sat in a Focus ST, I have sat in several Focuses, and for the size of the car the interior doesn't seem all that big. I didn't have much headroom in a car without a moonroof was something immediately noticed. While I did feel like I had enough headroom in a Fiesta with a moonroof, and it just got better with a Fiesta without a moonroof.
As for the price, you were comparing a fully loaded Fiesta to a base ST1 Focus... As noted, you can get a Fiesta ST with only the standard stuff for $21,400. It already comes standard with stuff that are options on the regular Fiesta like the automatic climate, My Ford Touch, interior illumination, leather wrapped wheel and shift knob, fog lights, push button start, and dual height hatch floor. Ford decided to add those things on as standard equipment because they though most buyers would already end up buying all those options anyway (likely based on previous sales of Focus STs). On top of that the Fiesta ST obviously comes with the 1.6L ecoboost and a 6 speed manual with 17" wheels, summer tires, with rear disc brakes, performance suspension, ST body panels, and all the stuff that makes a Fiesta ST an ST. You can spend about $22,500 on a fully loaded Fiesta Titanium with the regular boring engine and transmission with no performance suspension/tires/etc. Anything above the $21,400 price of the ST is optional, although the Recaro seats are recommended personally. The only real options are if the color is green or orange is costs $595 more, if you want navigation it costs $795 more, if you want the Recaro seats it costs $1995 which includes heated seats and side mirrors, Grey wheels and red calipers cost $375, and the moonroof costs $795 more. That's all the options other than accessories. They tend to add options to on the lot cars to make a bit more money off the deal. If you don't want the options, order a base car with a color other than green or orange and you've still got a great car for a great price IMO.
As for the price for a a Focus ST, the base Focus ST is $23,700. That doesn't come with the My Ford Touch system, the automatic climate, or ambient lighting. I'm not sure what other features it does or doesn't come with standard. The base price of an ST2 comes up to $26,095 and the price of a fully optioned out Fiesta ST (no accessories) is at $26,030 including either green or orange paint. Heated seats are not included in the ST2 package even though the Recaros are. All in with the ST3, moonroof, grey wheels, and tangerine color it comes to $30,115. That's over $5k more than the top dollar Fiesta ST. Yes there are a few things like projector headlights and a factory subwoofer that aren't even optional on the Fiesta (at least as of now) that you can get on the Focus. From what people say on driving the Fiesta vs the Focus, the Fiesta is a lot more fun and handles better, despite being down on power. I like the smallness of the Fiesta and as it has been said, it has a better power to weight ratio than the Focus. If you need the Focus because you need a larger car, then the Fiesta really isn't for you anyway. If size isn't important, the Fiesta is the winner in my book, and honestly I think it looks better. If interior quality is your deal, you're looking at the wrong cars altogether... Not sure what interior quality differences you saw from the Focus and Fiesta, but I just didn't see it TBH.
If you want a base car with no frills, the Fiesta still comes out cheaper than the Focus and already comes out the door with standard equipment that the base Focus ST doesn't have. But it really comes down to what you want in a car. Some people want Recaro seats, integrated navigation, grey wheels, and will pay the extra cost for green or orange. Shoot, I'd pay more for a factory subwoofer and projector headlights if they were optional, but I think Ford omitted them from even being option for the reason that you speak of exactly that not a lot of people want to spend that kind of money on a Fiesta. I know where an orange car is that has been on the lot since at least December, fully optioned and they've dropped the price to $23,995. I'm sure with some negotiating you could drop it further but that price alone is pretty good. You could find something like that and see where it gets you. Honestly, if I was able to buy a car right now, I'd be going to that dealer and making a deal on that very car because I really wouldn't mind a fully optioned Fiesta ST and I am planning on paying more than that for an orange one with Recaros and navigation, so why not get a few other options for free basically?