Keeping costs down.
Do organised events which don't use (so many?) commercial controls. Eating in cafes is expensive. Buy your food from shops instead. Maybe carry cutlery with you. Eating in cafes can be a godsend though, especially if it's bad weather.
El Supremo events are very good if you like his kind of event. He likes you to fill your saddlebag with his food so that you don't pay for a lot of food at M/Way service stations. Don't be afriad to stuff yourself on an El Supremo, he'll be offended if you go hungry.
400k plus, if you are fast enough and want sleep, book a hotel with a group and split the costs. Likewise for nights before and after events. This forum is very good for that. Book Travelodges in advance and you may get a room very cheap.
Book train well in advance via National Rail Enquiries for cheaper tickets to and from events.
Permanents, ride a DIY or a route which takes you past your home to use as one or several controls. Do a group event, visiting each other's homes for food/rest.
Ride a fixer. It's cheaper to run than a geared bike. Mike Kelly is riding more on fixed now because of ths, but his geared bike is a 10 speed
Makes a difference if you do a lot.
I probably spent around £6-7000 last season, but I did quite a lot last year.