At least $0.91.
The Kennedy half dollar is 40% silver so it might be worth more.
Correction
As of 01/2015 the retail value for a complete 1968 set from all 3 mints is only about $6. Denver was the only Mint that struck all 5 then-available denominations that year so a complete mint-state set of D-mintmark coins would be worth about half the total. As noted, most of that is contributed by the half-dollar which was struck in 40% silver.
Coin production in 1968 was affected by ongoing efforts to recover from the coin shortage of the prior 3 years as well as reintroduction of mint marks. Philadelphia didn't strike any nickels or halves, while San Francisco was pressed into service to mint circulating cents and nickels as well as proof coins. As a result Mint Sets contained an unusual distribution of coins: three cents, two nickels, two dimes, two quarters, and one half.
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The 1986 Uncirculated Mint Set sold for $7 in 1986 and today is valued at $16.
The "D" mintmark means that it was produced in Denver, Colorado. The terms Uncirculated and Mint State mean the same. The coin has not been used, shows no sign of wear from being in circulation, as is from the Mint.
If well-worn, about $6 regardless of mint mark. If almost uncirculated, 1940 quarters from Philadelphia are worth around $10, S-mint (San Francisco) coins go for about $15, and D-mint (Denver) coins are about $30.
There is no Denver Currency Plant. There is only two places that Prints Currency. It would be Ft. Worth Texas or Washington DC. The sheet my be a Federal Reserve Bank note marked Denver.
It wouldn't be a proof set if it was struck at the Denver mint (all proof sets 1968-present have been struck at the San Fransico Mint) and so it would instead be a partial mint set. I'd say it would be worth about $3-4 if it was sealed.