How much could cost to soundproofing the floor of 1 bed flat maisonette?

Something of an art. I'm assuming that you may not be required to comply with Part E building regulations. In that case, impeding the passage of sound can be dealt with by overlaying your floor with dense materials specifically made for the task. They will generally be small profile mats that are manufactured for application to existing structures to deaden sound transmission from below and will also reduce impact sound transmission to the space beneath. Your finished floor will be placed ontop. If you have access to the voids immediately beneath your floor then consider placing dense material here too - there are many varieties but look for dense fibre matting. For part E compliance - after having constructed a floor using a hybrid of techniques that just scraped through the sound test then I would recommend Robust Details. A web search on those words will tell you all you need to know. There are few straight forward remedial actions that will save you if the installation fails the test. Better to invest and get this right first time. And as for cost? This is not an inexpensive excercise at all.

for a good qualty of sound proof u looking at 900 pound

Best to google it and do your own research as there are a lot of materials and prices. Ring a few suppliers and talk to them about your job. Then make your own decision based on knowledge. I had to sound proof a music studio recently and the best info I got was "you can reduce the noise by 95% for a reasonable price, for 96% double your budget, for 97% double it again and so on". Educating yourself will masively reduce you being disappointed with the finished product. Good luck Michael