I asked one my fellow colleagues with experience in per diem. She said that agencies are given some room to negotiate salaries but not a whole lot. If you need a higher salary you might have to relocate to various areas that pay more, but with that usually comes higher living cost etc.

taxes
Yes you can in fact deduct cost of scrubs. Just don't go to crazy on itemizing because it may cause you to be audited. Make sure you keep all your receipts for at least 3 years.

You can right off any thing you purchase that is for work purposes. Keeping reciepts is key in case you get audited.