You can get midi controllers for less than $100. Maybe not with all the features of a good one, but you can still get one. Also, this one has the limitations of your computer's keyboard. For one thing, if you r using a cheap keyboard, which is not a gaming keyboard, chances are that only about 4 or so keys can be registered at once. To get the effect of being able to press every key at the same time, like you could do on an actual hardware midi controller, you need a keyboard with N-key rollover, so that all keys can be registered simultaneously. To further the limitations you would experience with a qwerty keyboard, simply being able to play solos with it is sort of limiting. While it certainly could be used for composing music, doing anything extravagant isn't going to happen. You also don't have the same precision of actual pitch and mod wheels, or knobs and faders. while software can mimic this, and maybe even to a passable degree, it isn't going to be the same experience as the actual hardware. So comparing the software version to a hardware controller is a bit of a stretch.
The only difference with this program that you don't get with Reaper's on screen keyboard is the mod wheel, pitch wheel and faders. You can, however, use Reaper's virtual midi keyboard in concert with an iPhone or iPad app. The two I use are TouchOSC, which I believe is $5, and Aftertouch, which is free. I also use Knob Lab, which is free to download but you will need to pay something like $2.99 to unlock all the layouts.
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