Overprint for "55 year of First Human Flight".
G. Titov (2010)
and U. Gagarin (2009) issue.
It is well know part of history (First Flight), and Russia Postal Administration know well how to celebrate it. This time, they just overprinted this two issues, and, made it "celebrity" in Philatelic Community and Philatelist well!
This two samples of overprint come in 2017 year, under Michel catalog number 2341 klb (G. Titov), and 2301 klb (U. Gagarin), with very "nice" catalog of 250.00 and 70.00 euros per sample.
I don't see U. Gagarin in Scott catalog update, just G. Titov overprint I see (Scott #7754), and G. Titov, single stamp with overprint, have also one good level (17.50 $ per single), U. Gagarin will be (probably) less valued, as quantity printed are much bigger, but, as this topic are very-very popular in Philatelic community, so some nice surprise can be (in catalog value).
Market? As usually, it is very difficult to say "stable" price, but somewhere between few hundred US$ per "superior" pack format, to 20-30.00 US$ per single (G. Titov), and some 40-50.00 US$ per souvenir pack, or 5-8.00 US$ per single stamp (U. Ggarin).
So, all depend of quantity printed around, but I think, that this issues will have "nice" future on the market!
Take it now, as smile in the future!