Ubervamp,
welcome to the Forum, I will try an answer some of your questions as best I can;
If you mean Infantry Regiments then the badges are going to cost on average from £5-8 for common English County Regts, £8-15 for the Regts that changed badges post WW1 (c.1920's) and £30-£40+ for some of the Irish Regts. The larger Scottish badges anywhere from £8-15. These are very average figures and you could pay more or even less if you shop around!
Dating the badges is often quite difficult as most of the above wore the same badges from the late 1890's right up to the 1950's!
As for Battalion variations, the Regulars mainly wore the same badges with only the Voluteers and Territorials having variations!
Some Pals Bns had their own badges, others did not!
Before you start obtaining badges, I would recomend that you get a copy of "Military Badge Collecting" by John Gaylor, which will explain the above in more detail and also has plates of images which you can tick off as you obtain them!
Hope this helps and as a bit of self advertising, I have a good range of WW1 Infantry badges on my Ebay site, all 100% genuine of course
!
Best regards
Andy