SPLA price increases - how do you charge your customers?

I guess this is more of a business question than a technical question.  I'm curious how other service providers are doing it and what they have found to be successful.  Our company is relatively new to the hosting market.  We typically sign a 3-year, fixed-cost contract with our customers.  This obviously has problems when Microsoft is allowed to increase their SPLA costs every year to whatever they want.  We include a modest "buffer" in our price to compensate for this.  But it's impossible to know if or by how much Microsoft will raise it's prices the following year.  Do you write that into your contract that the cost is subject to change depending on what MS decides what the new cost will be?  I do not see how a customer will agree to that.  So my question is - how do you do it?  Do you raise the price for existing customers when MS does, or do you eat the difference?  How can service providers be successful without knowing what the SPLA cost will be from year to year?