The RRP on these is $89, but SCA often have them for $59. The set includes: 6,7,8,10,11,12,13,14,15,16,17,18,19,21,22,24mm spanners housed in a metal tin.
Edit: buy through eBay and claim cashrewards: http://www.ebay.com.au/itm/Stanley-Combination-Spanner-Set-1…
You could do what Eric the Car Guy would do and weld on a larger size…
I don't think 9mm or 20mm are common (at least for cars)
SAE have agreed standards for fastener sizes which seems like a good way to go. In the metric world, 9,20 and 23 could be non-standard, but as you say, it doesn't mean you won't find a nut in one of those sizes.
Generally two hours into disassembly, under the car with oil dripping on your face.
Seems like the 9, 20 & 23 are missing. Probably why it's discounted…