Most of my cycling is done in jogging trousers, usually bought from TK Maxx, I stay away from anything too thick or cotton rich and they've all been fine, prices from £10 - 20.
I also have some Rohan Troggings, which are a cut above, but they're not cheap and rarely in their sales, I haven't ridden in anything else as comfortable when wet.
When I'm going to be out all day and just cycling, I'll wear Ronhill Bikesters or the Polaris equivalent, I'm not sure why, habit maybe, or wanting to look the part! When/if they wear out I don't think I'll buy any more, there's nothing wrong with them, just I find no advantage over the above (Well except needing to keep the ends away from the chain)