Don't buy ANY coin because it might go up.
But I really believe the mint and proof sets will be at far higher prices in the future and this applies especially to mint sets and more especially to gem mint sets. In the long run the demand for the coins in the sets is simply going to exceed their supply. This supply is so pitiful that even a modest increase in demand will wipe out the entire available supply.
Every year the number of these being made available on the market is shrinking. The original owners are getting quite advanced in age and fewer survive. Sets like the '68 were mostly bought by 30 to 55 year olds and these folks are all at least 70 now. Most of the sets this group has already sold has been cut up and the coins put in circulation.
There are no rolls of something like a '69 quarter so this leaves only the mint set and only about one mint set coin in eight of this date is nice. Most have severe problems.
Here's a survival estimate from another site;
It won't allow the link so here's the whole post. It's from August of '05 so numbers would be lower now.
Date.......Mintage.............................Surviving
1965... 2,360,000..............................450,000
1966 .. 2,261,583..............................600,000
1967 ...1,863,344..............................700,000
1968 ...1,980,000..............................350,000
DDO......55,000..................................5,000
1969... 1,727,000..............................300,000
D/D......90,000.................................12,000
1970... 1,838,000...............................500,000
Sm Dt.....200,000...............................60,000
1971... 2,193,396................................350,000
1972... 2,750,000...............................500,000
1973,,,,1,767,691...............................400,000
1974....1,955,981...............................550,000
DDO.......20,000..................................4,000
1975....1,921,488................................650,000
1976... 1,892,513................................600,000
1976....4,758,319(3pc-reg)....................2,000,000
1976.......150,000(3pc-high speed)..............70,000
1977....2,006,869................................ 450,000
1978....2,162,609.................................500,000
1979... 2,526,000..............................1,000,000
1980... 2,815,066..............................1,200,000
1981... 2,893,145.................................900,000
type "d".....16,000................................5,000
'82-P(souv)10,000....................................3,500
'82-D(souv)20,000....................................8,000
'83-P(souv)15,000....................................3,000
'83-D(souv)20,000..................................10,000
1984... 1,832,857..................................850,000
1985....1,711,000..................................800,000
1986... 1,153,536..................................500,000
1987... 2,890,758...............................1,600,000
1988... 1,646,204..................................900,000
RR 50c.....12,000..................................8,500
1989....1,987,915................................1,000,000
1990... 1,809,184................................1,000,000
1991... 1,352,101..................................700,000
1992....1,500,143................................1,000,000
1993... 1,297,431...................................800,000
1994....1,234,813...................................900,000
1995... 1,038,787...................................750,000
1996... 1,457,949...................................600,000
1997...... 950,473..................................650,000
1998....1,187,325...................................800,000
Surviving numbers are guesses and estimates of variety set mintages should be close.
Many of these surviving sets are in collections but the bulk are still owned by the original purchasers or were bought as speculation many years ago or more recently. The absolute quantities aren't as important as the relative quantities.
Some of the coins from the destroyed sets still exist in collections raw or in slabs, but large percentages have been put into circulation. These numbers are substantial, but it should be remembered that large percentages of the coins in these sets are very unattractive and will not even be considered BU. Choice and gem coins can be scarce and in many cases there is virtually no other supply than these sets.
Time don't fly, it bounds and leaps.