As others have said, rule no1, join a local club and you'll get a bucketful of races either free or cheap.
I joined my local club in May this year and have since done 30 races. The only "biggies" were the Great Manchester Run 10k (highly unlikely I'll do this again but I didn't know then what I know now
) and the Conwy 1/2 marathon which was actually pretty damn good for the money and I will happily do again.
Most of the others (Lakeland Trails aside) have been smaller, lower level club races, plus local league meetings, road relays, in house club handicap races and a couple of localish parkruns.
This weekend I'm having my first crack at running XC for my club and the cost will be minimal. We also have a midweek league Spring to Autumn on roads/trails with 6 local clubs taking part, plus a fell league and the XC league in winter. Add that to the monthly midweek handicaps and you can get an awful lot of competitive running in for less than a quid a week!
So as I said, join a club - if I can do it, anyone can 
Beyond that you just need to pick your events.
My own feelings are that the local clubs provide so much for runners of all levels that their events should be supported first, as without the clubs we'd only be left with the mass parcipitation type events catering for the "once a year charity runner" at increasingly extortionate prices.
I'm not saying these events don't have a place, but we need to ensure that they don't kill the "affordable" events for those that want to race regularly and I now include myself amongst those.
I don't mind the "commercial" races where the costs are proportionate (Central Lancs 1/2 in the New Year @ £12 being a fine example), but some events really are taking the p*** now and my only hope is that people start to see through them for what they are, a purely commercial enterprise taking as much as possible in return for as little as possible.
Trouble is, the once or twice a year "casual" or "charity" runner isn't going to care about that as long as they get a big medal to show how ace they are for doing their bit for charidee (starting to sound cynical now
)
To summarise:
1) Join your local club
2) support your local club events
3) shop around for the best value races
....and can we have more prizes made of local cheese please 