I've always wanted a solar solution to charge my batteries in the field but it's always been prohibitively expensive, much more expensive than an extra battery.
So I have gone with the extra battery, I now mainly use a 40Ah but have a 17Ah one too.
This dicksmith one is one of the most affordable I've seen, I think the previous quote I got was around $350, but I can't remember how powerful that was. That was from
http://www.12volt.com.au/ who are very experienced in such things, very knowledgeable and helpful. Actually I see they have a 5W for $135.