I think their cheapest prime right now is the 50mm f/1.4 which is currently selling for about $350. It's a really nice lens, and I bought it just a few years ago for $199 + a $50 mail in rebate. You can thank the current economy for the inflated price. That's about the cheapest prime that you'll find though. I also have the f/1.8 Limited 77mm which is currently going for around $800, again about $200 what I bought it for just a year ago.
Their prime lenses, especially the limited line, are all top notch and very high build quality. I wouldn't sell either of those lenses even at their going rate. You might check
ebay for a gently used one. Now if you go to looking at MF instead of AF, you can find some bargains. I also have the 50 mm f/1.7 in the AF which is pretty good as well and you can get that for about $25 on
Ebay.
All my other lenses are Sigma in the EX line, which is supposed to be their best. I've been pretty happy with those lenses but I may have just been lucky as a lot of folks complain of Sigma's quality control and many say that they have to send their lenses back several times to get a "good" one. Mine function well and are very sharp. I have the 10-20mm f/2.8, 24-70 f/2.8, 70 mm macro 2.8, and the 70-200mm f/2.8. Each of them go for around $500 as well except the 70-200 which is about $850 I guess now.
Now isn't the best time to find cheap lenses unless you find folks selling them on ebay to make their mortgage.