What is the minimum for participation in the pre-sale?
The minimum is set at 10 Eth! There is a feeling that the creators of the project want to weed out small investors) but this is just the pre-sale.
As you say this is just for presale. If you wish to contribute less then you will be able to do so at the main sale.
Of course, if someone wants to make a smaller amount, he can just wait for the main stage of the sale. And preliminary sales are almost always aimed at large investors