Not what you asked for but I've just bought straighteners from boots (own brand) for £9.99 and a vidal sasson curling brush for £19.99. I am really pleased with both of them.
I had my expensive ones for years but they both gave up just as I was short of money.
I have GHDs and they both straighten and curl (though the curling is in the technique, they're not curling irons) I'm not sure if this is what you mean? If yes, there are lots of simular cheaper ones out there that have the same shape so I presume they can also do the same thing.
i have a tresemme thing that has interchangable bits. it has a straightener, and 2 curling tongs and another bit. it was £25 from asda, but boots have them too. not sure how good it is as i went to use it the other night, and plugged it in to heat up, and it didnt get very hot. not sure if i left it long enough though. i was in a rush so just didnt use it. but it is a phillips brand, so i presume it would be good, if i was using it properly!!