According to this document: https://docs.google.com/document/d/1Xf_iHGI51Lp6nVHPZo0SdUw60-NA6YJUvk2s-eYaxLE
citation 1:
maxDeposit - the maximum deposit allowed to become a proof of stake validator. If you want to deposit more, you must purchase multiple slots. Set to 50000
citation 2:
Alongside this, another question is how to target the minimum entry size and a possible entry fee. One approach is to auction entries, thereby resolving many preference revelation issues and allowing targeting a fixed PoS participation rate, but at the cost of introducing or exacerbating selfish-mining attacks.
Looks like there is a talk about to make paid participation. And maybe there be a market of participation. With which, I'm, personally, perfectly fine and I think this is very good thing (see conversation in this question).
The path how I found it was: Ethereum documentation --> Other Ethereum Links: Giant Ethereum Resource List --> Proof-of-Stake Resources: Vitalik's CASPER Spec Google Document
Can't be sure is it relevant now.
So is there such a plan?