Have you talked to your VAR yet? Maybe they can make some type of deal with you to offset some of the cost. With 15 seats, that is a good size account. I would hope they would at least consider helping you out.
You should definitely contact them first.
As Jeff mentioned, the standard late fee is $500 per seat. So, if you could fly solo for at least 6 months, then from a strictly financial standpoint it would benefit.
Can you go without Upgrades/Service Packs?
For most companies, it is fine to use older versions of SW. Especially if all of your files stay within your company. Service packs are an improvement, but if you don't have issues with the features that are fixed, they aren't critical.
But, if someone sends you a SW2011 file, or you find a bug that needs fixed, you are completely out of luck.
What is the skill level of your users?
Is everyone very knowledgable and self-sufficient with running SW. Would any of them benefit from being able to call the VAR with questions, or to ask for assistance? Are there any new users that will need training/classes? Do you have questions about PDM? No maintenance means no support, so if you can handle everything internally, you could be OK.
The positive side is that you can always pay the late fee and get back onto subscription. But if you have an issue, you could be completely shut down until that is completed. I would expect it to take at least week to get a quote, po issued, received, and re-instate all of the licenses, ...
Being on Maintenance is essentially an Insurance Policy for SW. If you never use it, then it is 'wasted' money. But that one time that it saves you makes it worth the costs.