It is reported that travelers should have at least € 400 ($500) as proof of sufficient funds, including bank statements, ATM receipts, credit card limits, cash, or a letter of support from an inviting party. However, certification of these documents is not required. Besides Ukrainian border guards now have the right not only to clarify the purpose of the journey of a foreign citizen, but also to interview him/her to find out if he/she has sufficient funds.
In other words, it means that budget tourists will need more than just a passport to get in Ukraine. They will need hotel vouchers, letters of invitation, guarantee letters from the inviting party, or transit documents. The tightening of border crossing may be caused by a political confrontation with Russia and it may not affect the US citizens, but it will definitely affect the citizens from post-Soviet countries.