Hmm.
Would around £20-£30 be reasonable if looking at cheap things?
Here is a decent Mid Range which has decent specs and reliability.
Pyle PSPL05R - About 90-100 US Dollars.
...and here a budget range which has decent specs also but reduced in features =.
Pyle PSPL01 - About 25 US Dollars.
I have the PSPL0SR, and it's been a great sound meter. Very accurate and the features set are pretty good.
If you have a smart phone such as an Android or iPhone, you can also use a plethora of free apps which also provide you with a limited but decent sound meter.
Hope this helps!