If it's a culture lux then working the square early or even settling on it can be great just for the culture. Regardless, having a lux early is a no brainier if you can because the ai will swap a relic and gold for it in the ancient era, they treat it as worthless or worth 1 gold. If you spend the time each turn checking you will get a nice early faith generator, if you do not want the tourism, sell it for significant amounts in the medieval era to a low culture player.
As for just swapping it for 5-6 GPT. I am afraid I disagree that is not a lot. It's 2 tile expansions at a time you have little gold, those expansions give you an edge... has no-one noticed a little sentence whacker said in GOTM where he spends spare early gold on tiles. If we are talking efficiency, this is significant.
Seriously they will give a relic for the lux!!!!!???? (gotta try that next time as Kongo)... pretty useful for getting pantheon but don't wanna run god king and want the +1 to all cities instead... (although TBH I'd rather just pillage a shrine/holy site for that 25 faith lol... easier if you somehow find yourself in a war)
It doesn't even have to be tourism I think... with the faith from that relic, you can do a midgame switch to theocracy and the faith from the relic will itself be worth a knight at least.
As for buying tiles... I don't know about that... it assumes you are sure you can last long enough for the extra cogs and food and gold to pay off for your investment. By the time it comes into effect however, I think the game is already pretty much won or lost (around t40-ish... 30 turns after selling lux)