Reality check here guys, STM Discovery boards are cheap because they're subsidised by the sale of STM32 microcontrollers. You can't judge the price of any product that happens to use an STM32 just because ST sell a cheap development board to people wanting to learn and evaluate their processors.
What you have here is an independently designed product, which has to be profitable in its own right. Sale of one of these units does not lead to a design-in and subsequent sale of thousands of chips!
For sure, the box contains a processor, power supply and audio I/O that's compatible with guitar signals - which, I assure you, are a law unto themselves in terms of signal level, impedance and noise floor. The board has to be laid out, manufactured, assembled, programmed, installed in a custom designed, rugged enclosure, tested, CE / FCC certified, packaged and retailed in very small numbers. Then it has to be supported, and a few of them will have to be repaired or replaced. Add in the office overheads too - heat, light, power, administration, taxes, and the fact that engineering know-how is a valuable thing it its own right: people who have it have worked very hard to get it and have every right to be rewarded for their efforts.
There's absolutely nothing wrong at all with wanting to make "big money" as you put it, and just in case you're in doubt, virtually every product on the market is priced at what the seller believes the market will pay. There's no rule anywhere that says a product "must" or "should" be sold at a price dictated by the cost of the parts that went into it.
I didn't see a retail price, but if it's around the £250 mark then that feels about right. If the product is successful then they'll have a chance at a decent living - but they won't make "big money". If you think they might, and that they've not really done enough to earn it, start a competing business.