Generally speaking, off-the-shelf inverters are expensive for two reasons:
1) Power silicon, film capacitors, current sensors etc, costs money, quite a bit of money. the BOM for a kW level inverter can easily be in the $100's, especially in low volumes
2) Developing the hard and software for such an inverter takes time and effort. At low sales volumes, re-couping that investment is not easy!
Whilst there are several off-the-shelf BLDC software control packages from various manufacturers (ST, NXP etc) the problem with starting with one of these is you have no idea about the theories or basics, so when something (inevitably) goes wrong, you are often left with a smouldering pile of silicon and absolutely no idea what went wrong.
And whilst a $300 inverter might seem expensive, that's less than the cost of the oscilliscope you will need to buy to get your own version to work! (and let alone the soldering tools, power supplies, S/W toolchains etc etc)
If you want to learn about controlling motors, from a theory perspective, then an arduino and a basic H bridge (plenty to choose from on ebay) is actually a really good way of getting a small motor to spin on the bench. You'll learn a lot, and your 'failures' won't be too costly.