The tour operator can’t insist on you paying the higher price, but equally, you’re unlikely to be able to insist that the tour operator provides the holiday at the lower price.
The most likely result will be that the tour operator will cancel the booking and you’ll receive a refund of any money paid.
In these cases the first thing to do is to check when your contract with the tour operator comes into effect. This will be stated in the tour operator's booking conditions but it is usually when the tour operator issues their confirmation invoice (not when you received it).
If the contract hasn’t come into effect, then the tour operator simply returns any money paid and that’s the end of it.
If the contract has come into effect, the next thing to check is when the balance due date is. If this date hasn’t been reached, the tour operator is entitled – under the ABTA Code of Conduct and, usually, under the booking conditions – to cancel the booking and refund the money you’ve paid.
If the balance due date has passed, the tour operator won’t be able to cancel the booking but is likely to argue that the contract has been entered into on the basis of a mistake. If this is correct then the contract will be void and you’ll be entitled to a refund. You won’t, however, be entitled to compensation, nor to have the holiday at the lower price.
If you booked through a travel agent, it’s unlikely that the travel agent is liable for this as long as they’ve made the booking properly and disclosed their agency status and the identity and booking conditions of the tour operator.
If you’re planning to book on a travel company’s website, always check the booking conditions first. Some companies don't operate live websites and therefore you will be making a request rather than a booking. Once the company try and confirm the request for you only then will they provide you with a confirmation of your booking.