For an average charge of £9, you can already get a budget watch strap including fitting. Costs can be more than that depending on the shop. Mostly, big-named shops charge more, so if you want to save some money, you can try the smaller repair shops.
budget watch strap including fitting
Average Price: £9
Top Price: £23
Watch repair shops can change your watch strap for you for a small cost. They usually have watch straps available in-store for you to choose from.