Reasonable is quite a subjective term, by UK standards the major 5 star places along Sukhumvit (Westin, Sheraton, Marriot) are quite reasonable at around £50 a night (if booked through priceline) although thats pretty pricey by LOS standards. My personal favourite is the President Solitaire on Sukh soi 11 but they've really jacked the prices up in the last year or so, so now you're looking at around 3k THB + taxes etc.
The Majestic Suites on Sukhumvit near soi 4 and Nana Plaza is ok, the decor is looking a little tired these days and the rooms are quite small but the location is hard to beat and the staff are great. Not a lot in the way of facilities but if you're just looking for a place to crash its fine. Double rooms are around 1500 baht and up.
http://www.majesticsuites.com/
The relatively new Town Lodge on Sukhumvit soi 18 is excellent value for the price. I stopped at the Town Lodge for a night back in October, superior room was about 1400 THB a night including taxes and breakfast (although I was heading straight back to Pattaya so didn't stick around for the brekkie). Room was about the same size as the deluxe at the Majestic suites but with a lot better decor and has a DVD player in the room. The only drawback with the place is its right at the end of Soi 18, which isn't the shortest soi around, although the complimentary hotel tuk-tuk will take you down to the end of the soi and Soi Cowboy is just across sukh. They don't have a website but you can get more info here
http://bangkoktonight.com/hotels/town_lodge.shtml
