I would echo what Chuppster has said (you can spend as much on this hobby as you WANT) and I wonder if you've built any of the basic FT starter planes? Not only will it teach you how to work with foamboard but things like balance, CG and such.
One of my personal favorites is the FT flyer. It's ONE sheet of foamboard (Literally $1, well... Plus Tax.) You can find some really reasonable motor/esc combos (around $20) if you buy right from China (Banggood, Aliexpress) and you can pick up a FlySky i6 transmitter and receiver for under $50. I would recommend (if you are serious) to upgrade to the i6X (has better innards.) Pick up a couple cheap servos and you are in business. Batteries and radio are going to be the big ticket items, I would suggest getting a quality balance charger, but if you are really strapped for cash, a basic one will do (just keep an eye on it.) Oh, you'll need some props too (I usually run through a handful if I've got my friends flying with me, we're pretty hard on the planes...)
So, try a couple of FT design builds first before you try and design you dream ship. That will get you some air time as well.
I suggest;
Starting off with the FT Flyer
Once you've banged a few of those up build a Tiny Trainer (should fly with the same motor as the flyer) it has a slightly more involved build.
And try the cub when you are ready to move up (But there are PLENTY of mini designs to keep you busy for awhile.) You'll need a bigger motor but I've found it flies just fine on the cheap 2212 golden motors from china (they run about $5 each.)
Now, that said... You do get what you pay for... That $5 motor isn't going to be handed down to your grandkids or anything and some of the China stuff will really test your trouble shooting skills... Case in point;
https://forum.flitetest.com/index.php?threads/are-escs-consumable-items.53742/#post-414240
but I've had a lot of fun (and learned a lot) taking the route I did.
If you can, I do suggest maybe picking up a power pack from FT. Yes, it is a pretty big inital investment, but they are quality parts (well, except for the underpower 12@ ESC in the F pack... grumble, grumble.) AND you are helping FT share and grow the hobby (But that can wait, maybe buy a Power pack for that cub when you are ready.)
Good luck and I hope this helps.