I've aways been paid full rate (still £40/hour where I am) if one of my students was flying solo. You may also get paid a little bit if you're a QHI supervising a restricted FI - about £10 per hour. This can up your hourly rate a bit. However, that hardly adds up to anything regular, or never has for me.
Having tried both, I'd say instructing is a great part time job so long as you have a second string to your bow, but a lousy (and totally knackering) way to try to make a living...unless you're extremely lucky.