Microsoft Natural Ergonomic Keyboard 4000 It's awesome and well within your budget. Not mechanical, though. It's perfect for wrist strain as it gives your wrist a resting place. But if you have strain on your wrists, consider adjusting your table and chair height. Your elbows should be about even height with your keyboard.
Microsoft have a lot of decent ergo keyboard offerings. A friend with wrecked wrists swears by his Sculpt Ergonomic (within your budget and stocked by many physical stores so you can try it in person), although it acts funny under Linux. I have a Comfort Curve 3000 which is under $20. It's the only curved keyboard in this price range I know of.