clickyI quite often have to book cheapish accommodation at work, and find a simple google maps search is good for finding stuff like B&B accommodation. Although, depending on the area (don't really know Shrewsbury, so can't comment), you might find £50 is about as good as it gets. Certainly £30 is quite cheap even for guest house type accommodation - £40 might be more realistic.
The really cheap places don't tend to have fancy websites (if any at all) so you'll have to get busy on the phone.
Also, depending on when it is, Travelodge can do some pretty cheap rooms if you book more than 21 days ahead (non-refundable though). Might be worth a look, if it's not too late.