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If its going to be a one use item, maybe you'll be fine with not bothering to give it a price. Are the PCs going to buy it, or are they going to get it as treasure? If its is only going to be treasure, then it maybe doesn't need a price. Insert the item whenever the story makes sense.
When I try to make a new or unique effect for magic items, that no existing spell does, I try to imagine what the magic item would be like if it was a spell. Then I try to compare it to spells to determine spell level. After I determine spell level, it becomes easier to figure out a price. Then I determine what kind of magic item it'll be. For instance, A 2nd level spell in potion form is 300 gp because 2 (spell level) * 3 (caster level) * 50 gp (base rate for potions) is 300 gp. If it was in wand form, the base rate would be 750 gp, so the item would cost 4500 gp. If it was a use activated or continuous effect, then the base rate would be 2000 gp, so the item would be 12,000 gp. Because I've determined the spell level, I also know the minimum caster level required, so thats 2 factors covered, then the item type is the 3rd and at that point you are almost finished.
So with this spell, I tried to start with a 2nd level spell because the template raises the CR of the creature by 1. I'm not looking for a big change. Alter Self is a 2nd level spell that turns you into another humanoid, but the perks are limited to what humanoid races could get. It seems weaker than a spell that would turn you into a force creature. Going to 3rd level and you get beast shape 1 and gaseous form. Beast shape 1 is pretty mundane, but gaseous form is not, so it might be within the ball park. At 4th level, you get elemental body 1, which turns your body into a supernatural creature. The fact that small elementals are CR 1 helps as it puts in the range of a CR +1 template. So 4 (spell level) * 7 (caster level) * 50 gp (potions) is 1400 gp.
Of course you could decide I pushed the spell level too far and instead settle with a 3rd level spell. That would change the price to 750 gp, which is close to where you wanted to be at the start. Alternatively, you could opt for a weaker effect, so you could justify pushing the spell down a level.
Something to consider is that many of the spells I mentioned have a duration of 1 minute per caster level, so a 3rd level spell is going to last for at least 5 minutes. Would this be a problem for you? You wanted an effect that would last for 1 minute.
*TL;DR - I hope I didn't lose you with a lot of text