The cost depends whether you get things second hand or brand new.
125cc bikes (second hand) you can get for about £500 or over!
You can get brand new helmets from wulfsport for about £50 +
You can get second hand kevlars from about £50 + but it depends which rider you get them from and the quality of the kevlars.
The cheapest body armor is £50 or £60 but thats only for the top half of your body a full suit would cost alot more.
Goggles cost about £20.If they are roll offs they cost a bit more than that!
Gloves cost about the same as goggles but it depends on the make.
Hope this helps