Of course as you know it depends on the condition. I wouldn't spend more than $75 per seat, maybe $25 per seat belt, and $50 for carpet. Another option you have is swapping in custom seats.
i got quoted $45 and $35 a side for seatbelts from two different wreckers... and seats, im not too worried about custom just yet i just need to get rid of that hideous maroon interior ive got.