The motherboard CMOS battery is usually a CR2032. It is one of a family of coin-shaped lithium cells, CRYYXX, where (my guess) YY is the diameter in mm and XX is the thickness in tenths of mm. All nominally 3v, and thicker gives higher capacity.
The usefulness of a 10-year shelflife has wide applications; in my home there is now CR20XX in my camcorder (NV-DX100), central heating controller (Siemens), several calculators,... and more.
The replacement cost varies enormously. There is a CR2016 in each of the two key remotes for my car. My last garage invoice shows the parts cost of each to be (RRP) £2-61, less 10% =£2-35 plus 17.5% vat; so I paid £2-76 each. My usual source for Canon compat carts sells CR2016 at 30p. click here I bought a supply of CR2032's with my last cart order, now I see not listed,... not surprisingly. Before that I had oddments from Screwfix (ca £1 each), -only with other items to justify postage. I think Maplin's about the same price.
I didn't complain to my garage about the excessive mark-up because (i) The labour I pay for anyway. (ii) To swap my car key's CR2016 it must be done within 20 seconds, - which is limit for backup capacitor to retain data... slip up and I would pay for re-progamming. (iii) Key remote failure between services is entirely garage responsibilty. (iv) Garage bill over £400, so it's sensible to keep back quibble for bigger things.
I would like to see other links for cheap CRYYXX 's.