That one I couldn't tell you. You might try asking Gus @ riobravoreptiles.com, at least I think that's his site, what he thinks about it. Or PM Jimisnakes here, he's a boa breeder and really knows his stuff.
As far as Vet costs and all that, it varies to much to even try to tell you. Maybe call around and see if they'll give you an estimate on a general checkup and then one more involved.
Does the guy have any pictures of the snake when it was younger showing the lump?