Work out where the Expert should get instance pricing from

From the Expert demo. When the Expert walks someone through creating an instance, the price it quotes comes straight off the instance type. That isn't always what the team actually pays.

What we know so far:

* the instance type carries its own price, and that's what the Expert reads today
* some teams have custom prices set on the team itself
* others have custom prices coming from their plan
* the create instance form in the UI already does more than the Expert does: it prefers the team level price when there is one, and it factors in free allowances and trials

So the Expert can tell someone an instance costs X when they'd actually be charged Y, or charged nothing at all because they're still inside their free allowance.

Before we change anything we need to agree on where the price should come from and which source wins when more than one applies. @Esmewieringa could you help fill in the pricing sources here?