Well if we were to use the formula: For a use activated item (infinite times per day)
Spell level × caster level × 2,000 gp
Then it would be:
5 x 9 (req. to cast Animal Growth) x 2,000 gp = 90,000 gp
And for 4/per day it would be:
90,000 gp / (5/4) = 72,000 gp
And for 3/per day it would be:
90,000 gp / (5/3) = 54,000 gp
And for 2/per day it would be:
90,000 gp / (5/2) = 36,000 gp
Anything more than 5 a day is the same price as infinite use.
Each use would last for 9 minutes an activation.
This seems a reasonable price considering that it costs 18,000 gp for 1 use for 9 minutes a day as per the Ring of Growth from Savage Species.
Guess as "cheap" magic items go this is not one of them.