We've bought three cakes, same size and style.
First baker we've used for years as she started and grew her kitchen table business last year was £40 (with some discount).
March it was £60 (sorry we can't discount any more)
For end of May it was £80 in icing and £65 in butter cream) we decided that was excessive a d looked elsewhere. Met a lovely young mum working from home with a 5* hygiene rating etc. She knocked out the same cake for £50.
She reckoned the ingredients for the cake have gone up about £2-3 at the most this year. But I was one of many people who had come to her after seeing £20-30 rises from their usual bakers. To be fair to the girl it was far better than the last cake we got.
Original baker called chasing the order, we told her we'd sourced it for £50. She gave an exasperated fine and hung up. (We'd not given any suggestion we'd be ordering just asked for a price and didn't mention a date)
As for the OPs post the baker is wrong, I'd saying thanks but no thanks. The price was agreed when the deposit was made, both parties entered into a contract, she's now requesting a variation that you don't agree too. Either the agreed price stands or she returns the deposit.
Someone mentioned small claims, having been on both sides of small claims court and moneyclaimonline. It is worth noting it is unusual for costs (including the initial fee) to be awarded to a claimant (I got costs once on a spurious ridiculous claim we was defending). So while you could claim the £15 and be pretty certain of winning the costs would likely exceed what you'd get back. (And in any event I'd expect her to capitulate at the first turn). It may be worth following the guidelines up to the point you have to pay (formal letters, warning of court action etc) to see if you can get her to roll.
Hopefully it won't come to this.