I'd bet that especially for a country that thrives on trade that a definite win. Just think about the ports of Amsterdam and Rotterdam which serve as huge gateways for goods into the EU. They'd loose large amounts of trade if those goods could not be forwarded tax and border-control free to Germany and France.
I guess it's hard to estimate how much Germany gains from being in the EU. We're an extremely export oriented country and a substantial chunk of our exports go to the EU. Those certainly benefit - and those certainly benefit if the rest of the EU does better, so we could consider at least parts of the net payments as an investment. There are also other gains that are possible only because of the european integration: We're actually starting to share military assets with you: Germany provides the armored component of your 43rd mechanized brigade, the 11 Luchtmobielen Brigade is partially integrated into the Division Schnelle Kräfte, the german Seebataillon is partially being integrated into the duch Korps Mariniers and there's a LOI that the two navies want to share your most beautiful JSS, the Karel Doorman. Even from a purely financial perspective, not having to keep watch on the pesky Dutch seems to be paying of ;).
A lot of staff for trade negotiations has been cut, a lot of special rules and regulations from bilateral agreements were folded into common EU regulations. Joint research gets much easier, a lot of student travel/study agreements are governed by EU law.
Jokes aside: I think that a lot of the benefits that the EU offers go beyond the pure monetary part. Being able to travel freely, being able to work where you want without begging for a work visa. The Good Friday agreement in Ireland that basically upholds the fiction of two separate states on Irish soil while still having common cross border agreements is mainly possible because the relevant rules are done at a supernational level. I consider the money that Germany - and by extension me - pays a solid investment - despite all the numerous flaws that the EU currently has.