Generally see people doing them for £25 - £50 for a small business. Usually you can get other work with that, like printed stationary, or a new website, etc.
who's the logo for? if it's for dave the butchers, I wouldn't have thought much however if it's for Nike or Shell Oil. I'd probably be charging them a couple of quid.
The logo is for a law firm. It will probably be something that is the face of their company for years to come. Oh yeah and there is a website associated with this contract. The logo design came part of a side discussion I had with the client.
if web design is part of the contract - you'd better add it's (logo) value and then substract some percentage as a discount. It's good to mention this on an invoice.