I have checked and can confirm that this is a bug that affects subtraction of numeric constants from variables ending with an 'e' and when no spaces are used around the minus sign. As a workaround, add a space before or after the minus sign and it should work.
Sorry about this and thank you very much for letting me know! I'll fix this for the next version.
I just checked and (buyPrice)-2 also seems to work, I thought I already tried that last night but it must just have been alte and my brain wasn't working. Thanks for the help.