Note also that there are mainly two resolutions offered:
4 base scan is about 1500 x 1000 pixels and cheaper, about $12 per roll
16 base scan is about 3000 x 2000 pixels and costs about $18 per roll.
Also the quality of scans varies greatly.
When I first started scanning some of my old slides around 2002, I didn't know anything, just go to any colour lab, quality was so terok - eg very dark, colours off, etc - I had to re-scan 4 times before got decent (not good) quality.
See
http://www.art-photograph-gallery.com/photograph-of-hands-1.html for example of how much the quality can vary.
Recently my friend went to neighbourhood shop and same thing happened, scans looked terrible, as if my friend is bad photographer but his pixs are generally very high standard.
So I would recommend you spend a bit more and go to reliable place. After trying many places, I finally settled for Fotohub at Beach Road near steamboat places. Cost is higher but quality is good.
More recently, I found a friend who scans for me with dedicated slide scanner -- and with dedication. He puts his heart and soul to do a good job. Now I get not just good but EXCELLENT quality. But this will be costly because a lot of effort goes into it, not just straight scan. He also cleans my old slides, removes fungus, after scanning do colour correction with Photoshop, etc.
Those interested to see the difference email me at
richardseah@pacific.net.sg I will show you some before/after scans. World of a difference!