b19rro,
Unfortunately, there are a few factors involved in the pricing scheme and it is different for each vendor due to their discount from Mitel. It also depends on what exactly is being sold as well since if you only have a base system they may be trying to install a PEM [Processor Expansion Module] as well, and then there is the cost of doing the work and possibly upgrading the compact flash depending on the software version you are on. Without knowing your system and what is being sold it would be difficult to explain the price of the cost they are quoting you.
If I were you I would have the do a break down of the cost so you can do a better analysis of what you are actually paying for. Why are you going to upgrade to the PS-1 in the first place? Is this upgrade really necessary or is it to fix a problem?
Personally, I don't think anyone other than your vendor will really be able to help you with this unless there is a competitor in the area you can get a quote from for the .
exact same work and equipment
Sorry,
TE