Because Matarrita only has one year of MLS service, we only get 1/3 of the transfer fee. The maximum GAM is $650k from a transfer out. So as long as we get $1.95 million for him, we max out on GAM, with the other $1.3 million going to the league.
If he sells for more than that, and should, then we will get more money than $650k in GAM which must be spent in accordance with the following:
http://pressbox.mlssoccer.com/content/roster-rules-and-regulations
A few things.
First, the sliding scale based on years of service only applies to Generation Adidas players taken in the Superdraft. So, we get the full 2/3 of the fee.
Second, what we get 2/3
of is not the total transfer price, but the price net of the amount shared with his former club (the split is 50-50).
Third, as
NYCFCfan rightly points out above, the money we spent to acquire our 50% interest in Matarrita is not netted against the transfer fee when determining the 2/3 vs. 1/3 split, but it also does not reduce the amount we can take in GAM - which is up to $650,000 of what we receive.
So, taking your example. If the transfer fee is $1.95 million, we split that 50-50 with Alajuelense, which leaves $975,000. We then give the league 1/3 of that amount, or $325,000. We can use the rest ($650,000) as GAM, with the club eating the $325,000 it paid for him.
I think that's basically correct, but it certainly is confusing.