I would rather not mention the prices that I am buying at since they are so low (I am actually buying per XXXXXMB of RAM - not slots), but the biggest mods out there right now are bukkit and runecraft (off the top of my head).
Bukkit is a general mod that allows for administration and additional mods to be grouped up (ie: runecraft). You can look at other mods straight on minecraft's forums. There is a specific section dedicated to them!
Again, the easiest way to run mods is to allow the purchaser to add them. Give them an FTP or Control Panel, that way they can choose what they want to add or remove from the server.
As a number of other companies do, RAM is the biggest way to sell a minecraft server. Since you can set the server to allot XXXX RAM per 1 person's server. The lowest I have seen on this (less my place) is $10 for 1GB of RAM. I have a 20 man server at the moment running on about 1.5GB. Some have suggested 36people (comfortably) on a 2GB slot ~ $15 per GB of RAM.
What you need to do is listen to your customers. If they are saying they are lagging in the server consistently, you MAY need to provide more RAM to that specific server. Make sure you go into the servers every once in a while just to make sure everything is running smoothly (this is my opinion).
If you are looking for a list of server mods - the best known ones will be on the Minecraft Wiki:
http://www.minecraftwiki.net/wiki/Mods