I know that "affordable" is a variable criterion across the many people who enjoy this endeavor, so I'll toss out there that I got a hot plate at WM for $16. I also saw THREE toaster ovens with glass, bottom-hinged doors at Goodwill a week ago, even though I didn't need one.
I believe it was the venerable propane torch which got me into a heap of trouble a long time ago, with a significant pile of crap alloy. You don't know what the temp is, so you can easily melt wheel weights which are not lead. I know we're not talking about melting wheel weights, but wheel weights become ingots and ingots are like little clones and you cant tell one from another. Every once-in-awhile, one of those incognito bad ingots sneaks into a melt, and you MAY notice it when it doesn't quite melt like the others or you may not. If you use a torch to help it along, you probably won't notice it.
The hot plate has been the single biggest boon for me, using aluminum moulds - especially the big LEE 6C moulds, since I started casting. You can't get a 30-pack of Ol'-Mil' Light for $16, so I feel it's definitely worth waiting for and setting a couple bucks aside for. On principle, I try to spend the least amount and still get what I need - what works, and a $1 yard sale find would have made this an even neater addition to my tooling, but I don't regret the $16 one bit. Can't even find a used Lee 2C for that right now.