Many different dates & mint marks were counter-struck after they left the Mint with the Kennedy profile. Note, though, that none of these were official Mint issues; they were all done by one or more third-party companies for novelty purposes. While they retain their status as legal tender, this counter-stamping ruins any possible numismatic value associated with the base coin.
From 1971-1979 all Kennedy half's are face value only. The 1970d was the last 40% silver Kennedy made but was not released for circulation, it was a mint set issued only coin an that coins value is about $8.00
August 25, 2017. The 1970-D Kennedy Half Dollar in uncirculated condition has a value ranging from $6 at MS 60 to $5500 at MS 67.
That would be the 1970d with a silver content of 40% (ASW.14792) But they were not released for circulation and were Mint Set issue only. Some 1976 Bicentennial proofs were also struck in 40% silver, and since 1992 "Prestige Proofs" have been made in 90% silver. These coins were only intended for sale to collectors and were never put into circulation.
They haven't yet. Half dollars don't circulate well and some years and mints are not released for circulation because their not needed by the Federal Banking System. The years and mints not released for circulation are: 1970D/ 1987P-1987D/ 2001P/ 2002P-2002D/ 2003P-2003D/ 2004P-2004D/ 2006P-2006D/ 2007P-2007D
Your coins is either : 1) struck through something -- worth $50 or less 2) struck with low pressure -- worth maybe $100 3) been damaged or excessively worn -- no added value Try checking the links towards the bottom of my show schedule page to see if there is an upcoming coin show in your area, where you can take the coin and get it examined first hand : http://www.workingmancoins.com/ShowSchd.htm
25 cents. 1970 quarters are still in circulation and are not rare. The only ones that are worth any significant premium are quality uncirculated ones. As of 05/2010 prices range from about $1 for lower-quality uncirculated coins to perhaps $30-50 at the high end. 25 cents if in circulated condition, a bit more if the coin is proof or uncirculated. They contain no precious metals and are common finds in pocket change. They are worth a minimum of $2 and if in perfect condition $8 and up. Depends on the buyer. (for this specific 1970 US quarter)