As an investor you expect to bear diligence costs, especially
1) you (or your team) should have the significant expertise in your thesis sector to evaluate a deal;
2) an investment isn’t a sure thing - so asking a company to pay for diligence in advance is unethical;
3) with the possible exception of a specific key technology that is completely novel and proprietary which would require a specific expert to evaluate
4) additionally, if the deal is in demand you need to do my diligence quickly…