I'm not sure how much the cost will vary but a rough estimate would help a lot. Google doesn't help much!

Answer:
Really depends on where you buy it; at the market will be less expensive and they’ll fit it free but they are not jewelers so expect a bit of rough treatment and the battery might be well past its sell by date..

An electronics retailers such as Tandy (pricey) or Dixons (better) will have fresh stock and will be the ideal bet.

I should imagine the most high-priced will be a jewelers - simply because they’re expensive in all departments.

The price of the battery itself should not be high, less than £1.00 for some types (typically Silver Oxide) upto a few £'s for lithium.

You can get free delivery and really good prices on the first link below.