Why the dash block reward is shown as 1.8 as the normal block reward is 3.6. Does it means you take 50%?

Dash block rewards get split between the miners (via the pool) and the master nodes. The breakdown is as follows: 45% of the block reward goes to the miner, 45% goes to masternodes, and 10% is reserved for the budget system (created by superblocks every month).

Full details on the official site: https://dashpay.atlassian.net/wiki/spaces/DOC/pages/56655887/Mining+vs.+Masternode

Source https://www.dash.org/masternodes2/

Masternode Income

Masternodes cost money and effort to host so they are paid a share of the block reward to incentivize them. With current masternode numbers and rewards masternodes earn approximately a 11% return on 1000 Dash for the year of 2016. This reward falls off in subsequent years but as the value of Dash is rising and the supply of Dash goes down, masternodes should still be sufficiently incentivized.

What are you trying to say? 

It's directly on top of the very page you've linked:

Masternodes get paid 45% of the block reward on every block, which is distributed to masternodes one at a time. Typically, around 2 dash is paid to each masternode every 7 days.

Sorry

this is ROI

Apologize

No worries. We all make mistakes.