I'm not sure you can 'reserve' a holiday online.
Don't you have to go through the booking process and pay all at the same time?
From Monarch T&Cs they are saying you will pay the price that is correct AT THE TIME OF BOOKING.
That will be the price that is shown as you go through the on-line booking process.
They don't say that they may change the price from what it was when you made your online booking when they send out your confirmation invoice.
The only way to find out for sure is to ring or email Monarch Customer Services - and you risk losing your good deal.
One of the advantages of booking Greece holidays with Monarch is that you usually fly with Monarch airlines.
Olympic Holidays (after a major fall out last year with Monarch who had been their main carrier for years) used all sorts of weird and not-so-wonderful carriers such as Strategic, Tor Air etc and some people had awful 24 hour + delays as these carriers don't have the fleet that Monarch have and once an aircraft breaks down, they don't have a plane to substitute.