I’ve got a similar quandary for an iPad Mini 2, the button is failing (works if you press it hard) and I’ve been quoted $100 for the replacement (apparently it’s a whole screen). Same as you, they go for not a lot more than that, although you’re then maybe buying someone else’s problem so I’m left wondering if it’s just worth paying out the $100 and having my own known working iPad back.
Depending on what you do with it, maybe it’s time to upgrade, the 4 doesn’t run iOS 11 so you would be upgrading an effectively unsupported machine. You could try putting it up for $50 on Gumtree and see if some person wants to buy it and attempt the repair themselves and put that towards a replacement. I’ve seen iPad Air’s going for sub $300 which would get you an A7 + iOS 11 support. an Air 2 isn’t much more and gets you a little further. Mind you the current gen iPad 5 32GB WiFi is $469 (from Apple) so keep that in mind as you look at second hand units.
Another idea (which I have used) was to sign up with a carrier. With Optus it’s $35 / month for 10GB with unlimited Netflix and music. Sure it adds up to $840, but it’s essentially an interest free loan with monthly payments and you get 10GB of data (which for me adds to a wider data pool and means I could go for a base BYO SIM plan for one of our devices which helped offset the cost). Also it might make the Wife happy