Mystery Island is a great place to train quickly and pretty cheaply because you can train your hit points to 3x your level. If you do this properly, you can train more stats in each pay level before "graduating" to the next one. The most cost-effective training on MI is like this:
Level to 20 (one codestone)
Strength and Defence to 40
Hit Points to 60
Level to 40 (two codestones)
Str and Def to 80
HP to 120
Level to 80 (three codestones)
Str and Def to 160
HP to 200 (be careful here -- note that the next pay level starts 20 levels later, not 40 like before, so you can't train the max 3x)
Level to 100 (four codestones)
Str and Def to 200
HP to 240
Level to 120 (five codestones)
Str and Def to 240
HP to 300
Level to 150 (six codestones)
etc.
You could save some cash, probably, if you trained at Krawk Island for a while at first... but it's so much slower, and with them not having that "hit points to 3x level" rule, I don't know how much you're really saving. If I wasn't so lazy, I'd work out the math, but...
NOTE that you may not want to go exactly to level 20, str/def 40, etc., because you want to be wary of super bonuses. If you're training that last hit point and a super bonus gives you 121 instead of 120, it throws everything off. So perhaps leave one or two stats untrained (go to level 19, str/def 38, HP 57) in case of a bonus, or a faerie quest or a kitchen quest or random event.