Why do your calculations based on the highest fee for a premium, If you pay the annual, monthly fee is only $6 USD per month.
4096 for 12 months from Alliez and Tony: $25 * 12 = US$300 + US$82 = $382.
4608 for 12 months from Linden Labs: $25 * 12 + $72 + the cost of the land = $372 + land cost, call it $472, of which you hope to get $100 back at the end.
You get an extra 512m on the mainland, but you get more prims per square meter on the dAlliez islands, and you only pay for the land you're renting. Other landlords rent for less than tier, so there's better deals out there. I'll bet that if you wanted to commit to a year up front you'd get an even better deal.
Also: that L$24000 can go into Ginko and earn interest, and you don't have to front up the cost of the land and hope to get it back.