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Did you commission a pre-purchase inspection on the craft prior to acquiring it?
If so, the person responsible for analysing the technical records should have observed whether there was anything related to the main rotors which may have warranted their detailed inspection. If no work was performed on the mrb's and it is, as you suggest, a matter of corrosion, the challenge becomes one of investigating whether the blades were suffering from this at the time of sale and whether the seller was aware of their condition.
Even if both of these (potential culpability on the part of the seller) can be established there are laws protecting the seller once you have accepted the aircraft through purchase.
A tenacious lawyer with the support of capable technical advisers might be able to lay claim (in court) to compensation if it can be proved that the seller was aware of the blade condition - ephasis on the might however.
Regarding achieving this inexpensively .. not unless you have access to a lawyer who will offer you pro-bono services (challenging for helicopter owners, even if the copter is small) or your wife/girlfriend happens to be a lawyer (although you would then suffer loss of income from the time spent on this case). You could represent yourself if you are prepared to read-up on the specific issues involved or hire a low-cost lawyer and work with him/her in a scenario where you perform much of the research, investigation and case-building yourself.