The gun is not going to have any real "value" other than what one is willing to pay for it.
You can't look up a mutt build in a blue book and get a real world value for it.
An Essex frame from Brownells ran between $150-$180. I would shoot it before buying it. If it functions well I'd make a lower offer as the particulars aren't known. It could be full of crap parts or really good parts. Best advice would be to have it looked at by a gunsmith and looked over, especially the surfaces that make contact with the barrel.
I'd pay about $180-$200 if the gun/parts were OK but I'd probably just ditch the slide and have a dedicated frame for my .22 kit.
It's all up to you. If you want to have some fun and outfit the gun the way you want it with different parts it would make a great project and be a good learning experience.