Olga, I'm going to make the possibly incorrect assumption that you're in the US, but this information is relatively correct even if you aren't.
The most popular kits for an instrument which offers both pitch and volume control are the Jaycar kit and the PAIA Theremax.
The Theremax kit, with both the kit for the electronics and the kit for the case, costs about $200 US. Add an amplifier, and you've spent $300 US. Others can better discuss the strengths and weaknesses of the Theremax than myself.
Jaycar makes three kits. The cheaper one sells for about $50 US, but when you add shipping from Australia, it's about $75 US. It will require extensive modification to improve it to being an instrument someone might want to play, and even then it's still not going to measure up to the cheapest model from Moog. Also, you'll have to get someone to construct a custom case for it, because the case that comes with it is really much too small. (It's about as wide as my hand is long.) You'll also need to buy a power supply and an amplifier. You'll have spent at least $175, and you'll have an inferior instrument. (I'm not saying it's garbage, I'm making one myself, but it's not as good as the others on the market.)
The second Jaycar kit is their advanced theremin kit. It costs $150 US, plus shipping... let's guess it will come to $200 total, but I might be wrong, it might be more. Then you'll also need to have a larger case made for it, and you'll also need a power supply and an amplifier. You'll have spent over $300. I haven't heard from anyone who has completed the kit, so I can't comment on its quality.
The third Jaycar kit is a MIDI theremin, which is really a different beast altogether. I'm working on one of those too. It could be interesting, but I can't comment until I'm done. The expenses will be at least as much as the more expensive Jaycar kit, plus you'll need to use it with a computer.
So, even a kit isn't cheap. Buying the cheapest model Moog theremin, which is generally regarded as superior to any of the kits, is about $550 including the instrument, amplifier, stand, and audio cable. Yes, that's almost twice as much. However, I recomment you give it consideration: if you wait long enough to put together the money to buy the Moog instrument, you may be much happier with it.
If you decide to go with the cheaper kit from Jaycar, please contact me privately so I can give you additional materials on improving the kit, for the use of the person who assembles it for you. I can be reached via my web site at tomfarrell.org .