I think you need to look into the cost of BES Express given the advantages over using BIS as mentioned above. I don't know where you are located, but some carriers will subsidise the cost of this if, for example, you were taking out 5 new phone contracts. RIM states that you should run BES on a separate server, but the requirements would not be very high for 5 users. I support a BES Express for 1 user(!) and this runs quite happily on a server I got from eBay. This was relatively low cost as it included a Windows 2000 COA so I didn't have to buy an OS.
To be honest, BES was a bit of a pig to install and get set up, but once up and running it has been maintenance free.