We went round that way last year, had a couple of nights in each capital city.
Fuel, food and beer prices are pretty much the same as here now.
+1 for everything Tannerman said, if you don't want to go into the castle at Malbork there's a floating restaurant on the opposite side of the river.
Trakai near Vilnius has a nice castle, again, we didn't go in, just stopped for lunch at the pizza place across the lake from it.
In Vilnius we stopped in Pillies apartment. Right in the centre, lovely place, secure parking in a courtyard behind. Nice city.
Theres the hill of crosses on the way from Vilnius to Riga which is bizzare and worth seeing.
Most of the countryside is boring and flat. Most of the women are stunning.
Driving is generally shit, keep the eyes in the back of your head open.
Riga is ok, but not as nice as Vilnius, more set up for stag do's.
We stopped in the Two Wheels Hotel, secure parking, about a mile or so from the centre, full of Finish Harley riders who wouldn't lower themselves to speak to non-Vikings, but other than that good.
There's plenty of ex soviet ballistic missile sites dotted about the place if you find that stuff interesting, we had a ride round the one at keila-joa.
Tallin was our favourite city of the three, just a nice feel about the place.
We did the Air b&b thing for the first time for digs, didn't like it.
Hope that helps, have a great trip.