High quality printers actually have lower resolutions than you would expect. Nothlight Images have a good article on it here. The gist of it is that maximum image ppi (pixels per inch) at most needs to be 286 if viewing from 2 feet.

A 10MP image (2592 x 3888) from my canon 400D can be printed 8” x 12” at 305 ppi. I would also like to note that I have printed a 10MP image WITHOUT up-sampling at 30” x 40” that is 97ppi, If i get about 6 inches from the print, edges start to pixelate, so if I had up-sampled or had I high resolution image then it might have looked a little sharper. but then an image which is over 1/2 Sq meter is not meant to be viewed that closely and I would not expect them them to be super sharp when analysing with a loupe.

Round up of a few large formats for printing:

Size Trumps PhotoBox SnapFish
12 x 8 £2.49 £6.59 £1.20
16 x 12 £5.49 £6.49 £5.99
20 x 16 £9.99 £10.79 -
24 x 18 £12.99 - -
30 x 20 £17.99 £17.49 £14.99
40 x 30 - £24.99 £19.99

Although better deals are available, PhotoBox often run 2for1 offers and Trumps and other professional printers offer pro packages, which require you to sort out your files before hand. Pro Packages are a fixed price (not including VAT) for 1-10 prints and 11-20 prints etc, i.e. you pay the same amount for 1 print as you do 10. That could be 1 image 10 times or 10 images once.

For 20 24x18 prints you would pay £128.00 + VAT.

Price per image if printing 20 in Pro package

Size Trumps Pack PhotoBox SnapFish
12 x 8 £1.73 £6.59 £1.20
16 x 12 £3.04 £6.49 £5.99
20 x 16 £5.94 £10.79 -
24 x 18 £7.68 - -
30 x 20 - £17.49 £14.99
40 x 30 - £24.99 £19.99