How much would cost to develop app game like clash of clans

I dont think it would be 20k to make a game like Clash of Clans. At least 100k

I think you need to consider that the marketing cost can be even higher than the development cost.

I would like to say that the development cost of such a game is much higher than $15K, probably $100K is a more realistic budget. Of course it might be possible to create a Clash of Clans clone with $20K

Not That Only Guys,
Clash Of Clan is a online game.So it needs a very strong and very paid server which allows lakhs of players same time
Then in my opinion it should be greater in cost.Am i right?

Supercell (company) developed Clash of Clans around 2011-2012, I believe. Around that time, Accel Partners (venture capital firm) invested $12 million in Supercell.

While not a specific number in regards to the Clash of Clans games in particular, you can use that to create a basis for understanding about what the development, marketing, infrastructure, and general overhead costs were.