Kaliningrad is/was of considerable strategic importance to the Russian/USSR military forces, particularly as a warm water port. In the 50's the Lithuanian SSR refused the offer of Kaliningrad, and reunified Germany refused it in 1990, instead wanting payment. Why refuse this offer? How high were the sums that Lithuania and Germany wanted from Russia that they refused 15,000km^2 of land and what was the debt owed by Russia/USSR? Was it that worthless?
According to this article, two reasons:
http://balticstates.wikia.com/wiki/City_of_Kaliningrad
because of its heavy devastation during the war.
Lithuanians' concern to find themselves on equal ethnic-demographic terms with the Russian population within the Lithuanian SSR.