There are always taxis around, and if you book in advance you can get them for under 30 LTLs (7 quid). On arrival if there isn't competition you may have to pay up to 100 LTLs, but even that's 23 quid. This is from Wikitravel.
From anecdotal evidence, a friend and I were there in 2008 for 5 hours, took a taxi in and out and it barely registered as an expense so it can't have been bad :) The airport is only 6 km away.
From another website with anecdotal evidence, in the comments they suggest booking online and it cost them 9 LTLs just last year.
I suspect it'll depend - if you don't book, you're going to have to rely on there being some competition amongst the taxi drivers for cheaper rates. If you do, well then you'll know in advance. I suggest that option.
Or, you know, 6km is only an hour walking :)
More comments on this page highlight the difference in price when booking vs just turning up. You can really save some cash this way.