Everthing looks ok with your equn. to me except for the part about the current UP, or the nx part. Everytime you increase your UP, your cost increases your next up, so multiplying your current up by two after adding 15K to the current UP once wouldn't give the right answer. It's like compounding interest on the money in a bank; you're building on what you already had.
Directly from Deity's Guide to the Universe, located in the Neb/Faq fourn as a sticky it says:
Deity wrote:UNIT PRODUCTION
UP Upgrade Cost = 15,000+15,000F
.........................= 15,000(1+F)
............Where F = Unit Production Level (Which is just (Current UP-1)/3 E.g. if your current unit production is 583 per day then your unit production level is (583-1)/3= 194 which means that since starting, you have upgraded your unit production 194 times. The minus one is because you start out with a UP of 1 and this is divided by 3 because each level raises your UP by 3.)
Why? The initial upgrade costs 15,000 and each subsequent upgrade costs an additional 15,000 each time. So after 10 upgrades, it will cost 15,000 plus 10x15,000 (one lot of 15,000 for each subsequent upgrade).
Therefore Total Naquadah Spent To Reach Current Level is:
Total Upgrade Cost = ∫ UP Upgrade Cost dF
............................= ∫ 15,000(1+F) dF
............................= 15,000∫ 1+F dF
............................= 15,000(F+½2)
............................= 15,000F+7,500F2
............................= 7,500F(2+F)
Hope this helps.
edits: some done because wasn't worded correctly, and referencing

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