Slippery slope! It all depends on loads of factors,
eg new vs used kit and support mods, installation cost, shipping and duty costs, tuning options and tuning cost. Service and refresh of various components during and after FI, insurance etc
So estimate between 5k -10k minimum.
Yeah buying a custom FI'ed car will be cheaper but I would never feel easy unless I have proof it was well done or if not i'll take it down and reassemble to check every not and bolt and adjust things to my liking so it made add on the original costs and mean there is no real difference. FI is expensive and risky but the rewards are