Can recommend for 20/25USD
Safari Hotel St.126 (nice owner) with lift and larger rooms.

For around $50 There is 1 and 2 bed serviced apartments available only through booking.com at Seasons Residence Condo block just off norodom.

http://bit.ly/seasonsresidence


Or newly built Sun and Moon hotel on 136 close proximity to riverside seems to be around 60usd most of the time.

http://www.sunandmoonhotel.com
Only western quality hotel in phnom penh.