In reply to Kenny:
Nope. I think people who get married have a cheek to ask for anything at all - they should only provide a list if you ask for one, I think. There was some research or something saying it cost an average of more than 200 quid to attend a wedding! (Hotel, new clothes, transport, not to mention the present.)
Think your idea of a canvas is a good one. I did this for the bloke's Christmas present and he liked it. I took 2 pics he'd taken and was particularly proud of and got them put onto canvas. You can do it all over the internet and it cost me about £120 for 2. I would recommend the digital design studio (www.tdds.co.uk) as they were really helpful and quick and they put a picture of what your design will look like on the web (proofs) so you can see it, and they don't pressure you to buy it, and also they can do special effects like warhol style prints etc so you could get something really unique.
Alternatively a guy I know runs www.eyeswidedigital.com and I'm waiting on 2 prints he's doing for me. I can't tell you what they'll turn out like as not getting them till Sat but he gave me a really good price and I only gave him 1 week's notice!!!
I think people like it more if you get them something different rather than something off a list. Another wedding I went to I got trees planted in a wood for them. URL is www.futureforests.com - basically it's about being environmentally friendly and 'carbon neutral'. They get a nice gift certificate and card thing, and also you can get a tree sent to them. If you're into the charity thing then try www.greatgifts.org as you can buy something useful (from a water filter to a herd of goats) for a family in the third world on their behalf. The people I got this for were really touched (NB not everyone is as altruistic, some just want dosh/presents).
For random gifty weddingy stuff try www.tsena.co.uk and click on wedding gifts - they do really nice albums that are a bit different (handmade, suede ones are nice, or you can get ones with the names of the bride and groom on) or personalised wedding cufflinks or handpainted gift plates. The albums have gone down v well in the past as they are not your typical white cardboardy things. They fall within your price range.
Hope those ideas help. Personally I think it's a bit cheeky to ask for money because I don't see why they should know what you spent on them. They should just be happy that you're there and that you put a lot of thought into it!