Wondrous Item Creation - Help Needed

[5w337x7007h]

I'm an Alchemist and I sell my wares, at a discount to party members. I need a shop, but I've nowhere near the money for it.
My quandary comes from finding the price tag for my item. All I come across is spell x caster x 2000gp, and according to magic item creation rules, a spell can't be cast below it's minimum caster level requirement.
I did the math on a Wondrous item available in the Pathfinder Core Rulebook, Instant Fortress (pg.520), and it's base price is 55,000gp. Dimensions are 20ft squared by 30ft high, and that's twelve 10ft cubes. Those dimensions are equivalent to a caster level of 4 though a min 13 caster level is required.
Math for that item would be 7 x 13 x 2000gp = 182,000gp. But it's only 56,000gp if you go 7 x 4 x 2000gp.
The second equation would be impossible, according to the rules, unless you can cast spells below the required caster level, for the item's creation process.
Is there something I'm missing?

[rockdeworld]

Instant Fortress isn't the same thing as an item of MMM. Hence it explicitly says the cost is only 27,500gp to create, despite needing MMM to make the item. Then you sell it at double price.

For wondrous items, always use the price listed in the rulebook. For custom wondrous items (if you feel adventurous), work out the effect and price with your DM.

[5w337x7007h]

So if I talk it out with my DM there's a good chance it'll fly. He's really keen on custom magical items. Always talks about his past experiences with D&D and how one of his party members created a magical flashlight and performed the typical cop routine on a wagon his party managed to stop.