Agreed, these are going to be slightly over your budget but check
www.phacharasuites.com
www.dreambkk.com
check them on www.agoda.com and www.latestays.com and you will probably find a better rate than their direct internet rates. you can also use both of those services to identify hotels in your budget.
you want to stay in Sukhomvit .. . anyplace between Soi 2 and Soi 33 will allow you easy transportation via skytrain and access to Nana and Soi Cowboy . .