"Cheap" plane tickets are subsidised by the extras, notably notoriously on Ryanair.
Tickets aren't available a year in advance, so you are still booking three months or so in advance, along with everyone else. There is a limited number of "cheap" seats on each flight, and that could go in minutes, after the booking opens.
The "cheap" airports are still miles from anywhere; that is why they are cheap in the first place. Taxis and car hire, therefore, are going to figure in the costs as well.
With the pound's devaluation, food and drink on the Continent have got considerably more expensive, so the idea of a Continental holiday as "cheap" is pretty 90s/noughties; that is: outdated.
Basically, the further you are from home, the less you can control your costs. The bride and groom have to accept that people are entitled to control their own costs and decide the risks in their own budgets.
The first guest-friendly wedding I ever went to was in a village with its own station and a number of B&Bs, and a coach was laid on to and from the ceremony, so guest costs were limited to train, B&B, and lunch the next day. Interestingly, it was the first of the "second wave" of weddings - that is, the first not largely arranged by parents - so it was clear everything was the couple's initiative, and they used that initiative for everyone's benefit. As a skint twentysomething, I was really grateful, and tried to put those principles into practice for my own wedding, many years later; it made for happy, unstressed guests, and no-one was either overtired or feeling resentful for having to leave before being ready, just because it was time for the 20-minute taxi ride they ad had to book the day before!