How much is the cost to build Dapp on ETH platform?
It depends on the Dapp you want to use.
Dapp is a software. THere are softwares which are free, freewares... other softwares costs thousands of dollars.
Some complex software may cost millions, maybe billions of dollars to be produced (like AAA games). Soon we may have AAA games in the ethereum network, running as Dapps.
Other Dapps may be done in a few minutes and will cost nothing.
What kind of Dapp you want to do?
Is it cost effective to build Dapp on ETH than any other platform??
Probably Ethereum is the best platform, as it is the most tested and reliable. THere are thousand of developers working on it.