I usually stay at the Bopha Angkor which is a good 3 star that gets good reviews. It is in a good location and an easy walk to restaurants and pubs. Rooms are basic but clean and the pool is very nice. The deluxe pool side rooms are great but a little more than what you want to pay. http://www.bopha-a

Also check the Angkor Star Hotel which friends have stayed at and were satisfied. Angkor Star Hotel, Siem Reap

To view more hotels and compare rates this is a good site but be aware that to the initial rate shown you have to addd 22% for tax, service charges etc. Siem Reap Hotels - Discount Rates for all Hotels in Siem Reap