From the GOV.UK Website:
The Ukrainian authorities accept the following as a proof of sufficient funds: cash, bank cards along with bank statements, accommodation bookings, tourist vouchers, a letter from the person or company you’re visiting stating that they will cover your expenses, a return ticket or onward travel ticket.
The Ukrainian border agency website does not specify acceptable proof of funds; however, based on the GOV.UK info, a return or onward ticket should be accepted for this purpose.
Besides, no matter what some websites say, it is rare for them to ask a single question (in fact, most of them speak very little to no English). They just scan your passport, check the databases, possibly ask where you're going, and then stamp the passport (or migration Card if you choose to fill one out)