IF a witness runs multiples witness servers that will add up.

Let's do the math, the price varies where you host the servers.

$120 per month. 1 Witness node with a minimum 64GB RAM and 200 SSD harddrive.
$120 per month. 2nd Witness node for a backup, fail over purpose.
$100 per month. For Seed node. You can create multiple seeds if you like.
$650 per month. 1 Full RPC, self managed server. If you hire an employee cost should be added here.

$960+ per month with all these servers + labor cost (if available).

The Top20 will be able to afford this cost, but not all of them run Full RPC node anyway.

If you use Amazon or Google hosting in the USA, you will pay 50% more to host your Witness servers.

Will you ever consider a Witness? Answers the following questions first.

• Can you afford to spend $100 or more per month for the hosting fee?
• Can you manage the Witness server?
• Do you think people will vote for your Witness campaign? As of this date April 3, 2018, you will need 3200+ mVest (Votes) to be in Top100. The Top101 below produce 1 block a day which is equivalent to .9 SP, that is NOT enough to pay for the server.

If you still insist and still want to be a witness. Read this updated guide as of March 2, 2018
https://que.com/howto-setup-witness-node-by-yehey/

Vote your Witness wisely.

Thanks @berniesanders to open this discussion.