Those Kotobukiya kits can get pretty expensive, not that Bandai kits are all a super value. While I certainly have shelled out on kits $150+, most High Grade (smaller and simpler) kits are around $20-25, and Master Grade (larger and more intricate) kits are around $50-80. That's not even getting into things like tools and paint, like you mentioned. Gaming used to be my main hobby before I got into Gunpla, and that's cheap by comparison, even when taking a fancy gaming rig into account. Like any other hobby, though, you can do it on the cheap (simpler kits and basic tools) or dive into the deep end.
I know you said this isn't your hobby, but Bandai makes Entry Grade kits that are pretty inexpensive and simple to put together, no tools required even.