Party funding is a dark and murky issue. Each side has a vested interest in keeping the status quo as each side currently has access to significant funding from a small number of sources, i.e. Unions for Labour and business for Tories.
To be honest, I don't know what the solution should be. Labour should certainly fight this election within its means. So far I think I've made the party a profit (including the funds from my most recent fundraiser)