In Krause, the 1969 NZ Mint Set of 7 coins is $12.50 retail. The 1969 NZ Proof-like Set of 7 coins is $17.50 retail. There was no 1969 Proof Set.
You bought this from your Grandmother in-law.. Why worry if you lost money on this purchase? You made the woman happy didn't you? That alone is worth it! You shouldn't even consider it a gamble.
That's probably about what they sell for. It seems to me that the late 60s/early 70s sets sell for around $5. They get more expensive for some of the low mintage years in the 80s and 90s.
Not bad for $6.00, but I would have to see each coin. If the coins are really each PL, then you made out okay, but if not (if the coins are not all PL) then $6.00 would be just a fair price, no big deal. Another set is seen below, but it is produced by another mint (I think)? I do not think the set is all 1969.... the H.M.S. Endeavor coin may not have been minted in 1969. (the 50 cent piece). I cannot make out the other coins in your image, but the set is similar to this one below, which is selling for $32.95, which seems a bit high.