SPL at a given frequency for sealed (or IB) alignment is a function of displacement. Therefore, for budget minded builds, displacement per dollar rules. Still hard to do better than Fi IB3-18 there. One, two, three, sixteen, however many you need to get to your desired output in a given room is about the cheapest way to do it... sealed. Unless you don't need as much output as a single IB3-18 can provide, then look at cheaper subs. Keep in mind though that for any sealed design, larger enclosure (up to it becomes an IB) means less power and thus less cost required for amplification.
Ported can get you there with less expense in drivers and a larger enclosure. Here it isn't just all about displacement per dollar (still important) as you need resonances to be compatible, and the amount of power required to get you there adds to cost as well.
I guess if space was truly unlimited an insanely large horn(s) tuned to 7Hz might be the least expensive way to get to a certain SPL, at least for driver and amplification. Wood isn't exactly peanuts and the cost becomes significant when looking at massive enclosures. May ultimately be cheaper to have a much smaller enclosure with more drivers, or use an IB design that has an effectively huge enclosure without much cost.