Impossible to give you an estimate on cost because everyone is different, for example you can get a meal for £2 or £20 depending where you eat, some people drink all day and night which is not really cheap for a third world country, a small bottle of local beer is £2 in the main tourist areas.
Same thing with hotels, you can pay anything from £10 a night to £100 a night. To shag the girls you are looking at £20 upwards depending where you find the girl, beerbars are the cheapest option, GoGo bars are the dearest, bear in mind you have to pay a "barfine" to take the girl out of the bar (£6-£12).
The flight should cost around £600 for a direct flight, slightly less if you stopover in the middle east.
In my opinion 7 days is simply not enough especially if your flight lands on the evening of day 1, you should try to aim for at least 10 days but preferably 14 days which would give you a chance to see Bangkok for a few nights.
There are tens of thousands of prostitutes in Pattaya but the vast majority are rough as old boots, please don't take my word for it, go to You-Tube and type in "Pattaya bar girls" ----------- then decide for yourself.