To be honest, none. I think user testing from end users that have an organic interest in the product is much more valuable, and offering things can subconsciously sway how those users may approach use etc. If it was necessary to have enterprise eyeballs on it early, I would network my way into someone in that role and have them do it as a favor (perhaps in exchange for full access at launch)
Nothing - I work in an Enterprise company and as a Startup you'll be put through so much pain to get on their systems and it'll take so long that it would be a bad deal to pay.