Unfortunately, it's hard to answer this on the forums. I'm sorry for that but there are many variables.
As for the user costs, it depends primarily on the icensing model you pick - ex: CUWL Basic, CUWL Std, CUWL Pro.
In addition, the list price will be one thing but the price extended to you by your reseller will likely be lower.
You need more than 2 servers for 3500 users. Once you get over 1000 phones, you need a dedicated Publisher, TFTP Server, and a minimum of 2 Subscribers (1:1 redundancy) - this is specified in the SRND for Cisco UC.
You may want to check with your Cisco AM or trusted Partner to help you spec this out.
You can definitely provide services to 300 users with a standard CUCM cluster - Pub and Sub. The Sub will be the primary call processing node and the Pub will be the DB/TFTP and can be the backup call processing node if the Sub is offline. No problems there.
Another option for you that provides some additional bang for the buck is Business Edition. It's CUCM BE and you get a single server. The MCS server from Cisco is sold as a specialized 7828 server that has both CUCM and Unity Connection (voice messaging) coinstalled and supports up to 500 users. My only issues with BE are that there is no redundancy capabilities. 1 server solution, that's it.
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