I'm a nightmare for checking for a bargain, I'll have Agoda, Booking et al, all open on different tabs, including the hotel direct, plus Topcashback too, to see if any of the sites have any cashback offers. For my upcoming trip, I have a saved enough for a few ST, even if the cashback will take forever to come through 

Use Trivago: https://www.trivago.com/  It will give you a view of the prices. Usually Agoda, Trip.com, and booking.com give you the lowest prices. If you definitely want to stay on Soi 11, try NISA hotel: https://www.nysahotelbangkok.com/  If you do not mind to walk a little bit, opposite Soi 11 is Soi 8 with several hotels. I like Adelphi Suites there: https://www.adelphihospitality.com/destinations/adelphi-suites  

Keep an eye on prices. They often drop if vacancies remain high. Sometimes I find booking direct is actually cheaper than going through one of the consolidators like Hotel.com or Agoda. It's always worth checking them all.

I've found Agoda usually has the most competitive prices, they're also owned by booking.com but they usually have better deals for SEA. Also contacting hotels directly can sometimes net you a discount off of any publicly advertised rate, I contacted Maison Stay on Soi 4 directly and they offered me I think what would be like 10 AUD less a night than anywhere else I could find, plus late check out.

Try Trip.com. When I compared the rates given on the hotel that I eventually booked, Trip.com quoted me lower prices than Booking.com. It might not be the case for you but it was what I noticed. I hope this will help you.