Local councillors aren't really paid at all, even if they're successful - they get a small stipend. Councils therefore end up getting mostly run by their permanent officers. Council candidates are almost entirely volunteers. They will get election material and some staff time from their party, but for being a candidate you don't usually receive anything. They can solicit donations, but as you can imagine, there is not a lot of money in local councils.
Elected MPs get paid 80k, which is good but not excessive for a long hours managerial job in central London.
MPs are similarly volunteers, but because of the time commitment for campaigning they tend to be limited to people who can take a few months off at their own expense. So you see a lot of people from freelance/self-employed/small business backgrounds, such as lawyers and management consultants. Some are also "journalists" such as Boris Johnson, who is paid £275k for a weekly column by the Daily Telegraph. Or Seamus Milne, who moved from being a Guardian journalist to Labour Party Director of Communications.
Another category is people who are already party or trade union employees. They usually get to arrange to keep their job while campaigning, on reduced or no duties. MPs have assistants who help with research and constituency work, paid from MP's allowances. This is an excellent job for someone who wishes to become a candidate, as you get to see the job from the inside and meet relevant people.
Party turnover tends to be that of a medium-sized business. Enough to make a comfortable living for the senior members, not enough to pay lots of unsuccessful candidates.
(I believe the same applies for AMs, MSPs, and MEPs.)
Is there an exploitative bottom end? With lots of young hopefuls putting in the leg work in the hope of making it big one day?
Pretty much, although given that the high end isn't really making much either it's more a kind of self-exploitation.