How much do the lockers costs on a 24h basis?
Unfortunately not likely the current price but may be of interest. From Greg R on TripAdvisor:
4. Re: Coin locker or left luggage in Tbilisi anywhere 29 July 2013, 19:30
Yes, the one in Tbilisi charges 4 lari per bag, for 24 hours I think. I used it about a month ago. The station itself doesn't provide a left luggage service, so this gap in the market is filled by a small business. It's located on the first platform, further along the tracks from the terminal building in a white portacabin. Although it looks rather scruffy, it is well used and they're trustworthy; just don't lose the ticket, because they speak no english at all. It's open 24-7, and I think the family that runs it live there.
If you're on the first platform and, for whatever reason, can't find it. Ask one of the many people who will be hanging round there: Gdye komnata dlya chimadana? if they don't speak english?
There is one more new Luggage Storage in Tbilisi Center at Liberty Square #4 (next to Courtyard Marriott in arch).
It's more comfortable because it's self service and you can use it within 24 hours.
Hope it's helpful information for travelers!