If I recall correctly the 5X7 are about $9.00, the 8X10 are about $20.00. They usually print the casual ones in 5X7 and the nice formal ones in 8X10. If you want other sizes, you have to buy the ones on display first. It is a great money maker for the cruise lines but some pictures we just had to have. We cruised just before Christmas, so we bought a package with additional sizes, purchased nice frames when we got home and gave the photos to our parents for Christmas gifts. Wouldn't have bought the additional sets if it wasn't the holidays.
Photographers will be everywhere, especially on the formal nights. My husand and I had our photo taken every chance we got hoping for one or two good ones. The only thing it costs you to have the pictures done is time, some lines can be a bit long but you only buy the ones you like or none as the case may be. Our town teenage daughters made some friends (there was about 6 of them) and took some really cute shots. We purchased a few of these as cruise momentos for them since they really didn't want anything else. Can't recall the prices, not cheap but not highway robbery either. It was fun getting pictures taken - it's something we never do except snapshots on holidays or birthdays.
Also If you want photos done try and do most of them at the first formal night, because by the second formal night you have a lot of sun on your face ,so either racoon eyes from your sunglasses or a fed nose ect.
the photographers are everywhere and you can get carried away wanting to buy this one and that one. Don't go nuts, pick ones that you totally love --definatley get a formal portrait picture taken, go early so you don't have to wait in a long line, if you are a group, you can make reservations for your pictures.