i've stayed in a safe, clean and comfortable guest houses in angkok for exactly USD15. and USD 15 was for 3 person altogether!
came with a tub, hot water, air con, TV and great views. no breakfast and mini fridge tho.
the more expensive hotels are no doubt more luxurious, but they are usually further from the centre of town. which is the Bar Street area. so you end up paying more to get a ride into town.
i cant refer you to a website, cause usually these cheaper guests houses do not list themselves online. you just need to snoop around for it. plenty of these guest house around i assure you.
not to worry about reservations, June is the low season. should have plenty of vacancies
have fun!