How much is this worth & how will it perform?

[Atra1n2]

So, I've been looking to purchase a cheap, pre-owned  pre-built and add a couple upgrades and make it a decent PC for entry and mid level gaming. One in particular I'm looking at is a SFF PC with a i5-3470 (quad-core), and 12 gb ddr3. The kind of upgrades I would do is to throw in an SSD for the OS and leave the HDD for mass storage, and add one of those SFF gtx 1050s, like the one from MSI that does not require supplemental PCI power. With that in mind, how much should I be reasonably willing to pay **USD** for such a pre-built, and how much would it be worth after making the upgrades? Also, would this be a decent performer? I haven't had a lot of experience with used PC parts, but I think the 3470 should still be a good performer for entry & mid level pc games. If this idea is a good value proposition (performance/dollar), I'd like to do this and give it to my nephew who is beginning his venture into PC gaming.

[Damascus]

Max 100 or so usd depending on the hdd size and psu capacity.