Re-Negotiation? on property offer

[Dee Culverson]

Looking at a property with 3 rentals. Store front, 2 apartments. Rentals pay mortgage and provide some income.
Had an inspection done and negotiated what I thought was a good deal, 300k (about 10k renovations needed) but after further inspection come to find will need about 30k renovations. 
Do I go back with additional estimates and try to get the 20k difference off our handshake agreement? (nothing on paper yet)
or do I eat my mistake (property will still make money) but I am now in the hole an additional 20k to start?

Mostly wondering if anyone who has been in this or similar position successfully re-negotiated a deal and how it was done.

Thank you!