What would be an appropriate Icon that could be used for Subscribe or Subscription. A subscription meaning; where a customer must pay a subscription price to have access to the product/service.
The problem with having a subscribe button without the word 'subscribe' is that there is no standard icon for this. The only subscribe icon I can think of is the RSS feed subscribe button:
which obviously wouldn't work in your case.
To answer your question though, there is no standard icon for this, so you should probably include the word 'Subscribe' or 'Join' wherever the button appears if you can, because it's just not something people will recognise if it's just an icon. If appropriate you could use the shopping cart icon, but this doesn't usually make sense unless you offer multiple products, and may be confusing for something you pay monthly for. If you happen to include an icon along with the text, and do this consistently, it may be OK to use just that icon in some places.
If this is an important call to action (if it's the way they buy your product, it should be) you need to make sure it's as clear as possible.