I agree with Turussa (although many Audaxers carry spares, and I seriously considered it).
For the extra weight of the spare, you could use sturdier tyres on the rims. And if you use a really cheap spare, you run the risk of ruining that one too!
So the tyre boot seems like the best all-round, statistically risk/hazard/probability assessed, strategy.
I think.
p.s. I think Mike Dyason does some stupidly cheap tyres, if you insist!