Green’s Dictionary of Slang

the highest price or rate possible.

1942		Birmingham Mail 13 Feb. 3/3: [advert] We don’t want junk but for anything up to the mark we are definitely paying top whack.
1956		Leicester Eve. Mail 4 May 21/3: [advert] REMEMBER YOU MAY PURCHASE ANY OF THE ABOVE [i.e. cars] WITH ABSOLUTE CONFIDENCE. AND KNOW YOU ARE SAFE. TOP WHACK ON THE OLD ‘UN.
1966		Dly Mirror (London) 26 Feb. 23/7: Why shouldn’t the crowd show their dissatisfaction when they find they have paid top whack to see an experimental team serve up a load of rubbish?
1976		Drive Nov.–Dec. 50/1: The 1.3s have a reasonable turn of speed, too: top whack is 86mph [OED].
1986		F. Taylor Auf Wiedersehen Pet Two 53: Top whack for us, though.
1993		(con. 1945) S. McAughtry Touch and Go 51: I’ll pay you top whack.
2001		K. Sampson Outlaws (ms.) 90: Some of these lads here though, they want to pay top whack. It’s the big I Am and that.
2009		T. Black Gutted 10: Those Badger Protection boyos are paying top whack.