LCDs tend to come in these sizes.. some aren't too expensive and most have discounts frequently through Dell. Their lower models have the same resolution and screen size, but usually give up some contrast ratio, response time, and viewing angle for reduced prices.
1024x768 (standard 4:3 15" & 17")
1280x1024 (standard 4:3 19" & 17") ~$200 for a 19"
1600x1200 (standard 4:3 20"-22")
1440x900 (widescreen 16:10 19" & 17") ~$170 for a 19"
1680x1050 (widescreen 16:10 20" & 22") ~$215 for a 20"
1920x1200 (widescreen 16:10 24" & 27") ~$375 for a 24"
2560x1600 (widescreen 16:10 30") ~$1050 for their original 30" (the new model is $2k)

LCD prices have dropped so much in the last few years. I bought my 24" Dell 3 years ago for just under $1000, and that included a discount. I picked up the $215 20" a few months ago and it's a fantastic monitor for the price. There are plenty of other manufacturers out there with even lower prices - I just used Dell's listing since they have nearly every monitor size available, are of very good quality for LCDs (I believe Samsung and LG make the panels), and aren't outrageously priced.