after overhead, a masternode earns approximately 15% or 150 dash per year.

so 10 masternodes would get me about 1500 dash per year?

You would have to own a majority of masternodes in order to attack instant transaction locks. The cost of acquiring this many nodes (currently about 1.7 million DASH) would be prohibitive, as there is obviously not that much available for sale and even if there was, the price would skyrocket if anyone tried to obtain this much.

if the NSA is interested in "attack" DASH is because their volume is high, so the price they pay dont be 1.7 million, maybe 170!

Please someone explain this

I was thinking this morning about the differences between Satoshi's work and Evan's. Satoshi built on the work of HashCash and others, while Evan obviously built on the work of Satoshi. Both are brilliant and made (are making) extremely important contributions to the field:

Satoshi had the idea to use blockchains and proof of work to solve the Byzantine General's Problem. This made double-spends (the bane of all prior attempts at digital currency) impossible without the collusion of a majority of miners.

Evan built on this idea but went further, creating a two-tier network. He used this two-tier structure to solve the problem of sharding. Proof of work blockchains are necessarily slow and can only process relatively few transactions per second, because every node on the network has to process every transaction. Sharding solves this inefficiency by using quorums of masternodes to "lock" transactions while they wait to be put into a block. Evan has done it with InstantX, and is now working on making all transactions essentially InstantX by default.

so 10 masternodes would get me about 1500 dash per year?

Yes.

Here the evolution of the % (theoretical) with MN number :