I just put together a similar prototype for under $10. I just bought some sheets of small colored circle stickers from Amazon (https://www.amazon.com/gp/product/B016LDXXJY/) and used dice I already owned. I printed my symbols on the stickers and then stuck one to each side of each die. And of course you could just draw the symbols yourself.

It's not ideal, the stickers overlap a bit on the edges... but it's simple, cheap, and certainly good enough for effective playtesting.