Masternodes are around to support and secure the network, they ping traffic around and provide some unique cryptographic features, they facilitate the anomymization of funds and enable double-spend-proof transactions through lockdowns. They are vital to the smooth operation of the infrastructure and tend to be highly lucrative!
Anyone can run a Masternode providing you got the coins, reliable hardware and a rock solid internet connection, ideally you are aiming to be up 100% of the time. For your service you are rewarded in most cases quite generously as much as 45% of the blockchain reward.
Its common for Masternode owners to rely on the rewards as a regular stream of income.
Examples:
Stratis (250,000 Coins Needed) https://stratisplatform.com/2017/04/13/stratis-announces-first-master-nodes/
Dash (1,000 Coins Needed) https://dash-news.de/dashtv/#value=1000
NEM (3,000,000 Coins) https://blog.nem.io/supernodes/
PIVX (10,000 Coins Needed) https://pivxmasternode.org/
Waves (10,100 Coins Needed) https://wavesplatform.com/
Potential Earnings?
Lets take a look at a PIVX node (Last Updated: January 23, 2018, 10:40 pm UTC)
Out of your scope?
There are many cheaper options which i'll be exploring and blogging about, you can run a Masternode for under 500 dollars, not to say they are any good though but its certainly worth exploring, doing your own research and choosing a project that you believe has a genuine role to play.
Watch out for level 2!
Some great links:
https://masternodes.pro/
https://masternodes.online/