I will answer you in terms of DASH ROI, the value in FIAT will of course depend on the price.

Requirements

In order to run a Masternode, an investor needs to buy 1000 DRK/DASH as collateral. These coins remain in control of the investor, but he needs to put them into one of his addresses and associate that address with the node.  The network checks that the node is running and that the funds are not moved, this classifies as a valid Masternode and then it is eligible to get paid.  With each mined block on the network a Masternode from the active Masternode list is selected and gets a payment, when all masternodes on the list are paid, the payment cycle starts again.
When there are updates in the software the Masternode operators have 7 days to update to the latest version, after that, they are dropped from the active Masternode list and stop getting paid until they update.

DASH ROI

You can estimate like this:

- Currently Masternodes get 40% of the block reward.

- The minimum block reward in DASH(DRK) is 4.65 DASH at this moment. I will assume difficulty is high enough so that we are dealing with the minimum reward for a worst case scenario situation, sometimes there are blocks with higher rewards. The minimum DASH block reward decreases by 7% annually to avoid abrupt block halvings, that already happened earlier this year so 4.65 DASH will be valid until next year.

- Average block-time in DASH is 2.5 min, so there are ~576 blocks per day. So 576*4.65DASH*40% = 1071.36 DASH alloted for the whole masternode network currently, we need to divide this by the 2300MN active masternodes. This gives us

 ~Monthly masternode income = 13.97 DASH
 ~Current Minimum DASH Annual ROI = 17%*

*This will change and go up throughout this year as the masternode network reward share will continue to increase each month to incentivize node creation following this schedule:



IMPORTANT:

-So in the end it will really be more than my estimation above as the reward share will continue to increase each month until it reaches 60% March 2016.  

-Having said that the number of Masternodes will also likely increase so I think it is safe to say tha the minimum annual return for a node in DASH should be around 20-25 %

-Now the return in FIAT could be a lot more if the coins appreciate in value year to year.

- Finally, and very important you always retain control of your 1000DASH and are able to liquidate them at any point you wish, you would then obviously just stop getting paid.

I really hopes this helps.

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