Well said something before but I'll say it again I'm not charging for work but when I do, I'm not busting a hump for less than I would get working at Poundstretcher. Regardless, as was said, of how talented you are. If you are working you are doing it for a minimum of £6 an hour (and most say train for 3 years and you are there you are saying you've done it for 4...so I mean what is going on~?).
I'd work for a plasterer for free but the difference is I'm not working for nothing, I'm working for my education at the minute you are acting detrimentally to the trade if you are working for peanuts for something you've worked 4 years for. If you are out on your own...its been said man sort it out. Glad to hear you got a higher price. keep it up.
(minor addendum)
Would you work for £30 a day after 4 years training for Argos? Would you train for 3 years shovelling **** or working at Asda for £30 a day? Just how **** is plastering that you can train for 4 years but still not consider yourself worth £150 a day. I am 32, if I've got to go till I'm 36 before I get to £40 a day well fortunately I know the hangmans knot if someone can lend me a thick bit of rope....