A couple things:

A. You really shouldn't go picking your own shrooms unless you know what you are doing. Liver failure isn't worth the possibility of tripping balls.

B. Too lazy to convert American dollars to Euros, but around my area 3.5g (an eighth) is usually between $25-$40. The main issue here is who you know. As with most drugs, the closer you are to the "source" and the better friends you are with said source, the cheaper your drugs will be.

C. As others have said, you might want want to start off with a smaller dose (i.e. between 1-2g, 2.5g at most). I know you said you want to "trip balls", but you honestly don't know what that means, being that you haven't done it. While there is the school of thought that one should just jump into the deep end to begin with, and have a full psychedelic experience, I for one believe starting off small and working your way up is the best course of action. Psychedelics are a beautiful thing, and you wouldn't want to be turned off to them because of an overly intense first experience. Some people are very sensitive to psilocybin (I had a friend who would be absolutely floored by 1g of good cubensis).That being said, do what feels right. If you have done a good amount of reading on the subject, and you feel you have a strong, confident mind with no proverbial skeletons in the closet, go for it ;)

Good luck, traveler.