Most recently, I started modern with a pretty reasonable gold production going into modern (600/turns), 4 silver, and left 2000 gold in reserve to buy my first explorers, and I could tell new cost throttled getting more out faster. I also hadn’t explored the DL in advance and so had to use explorers, my navy and a few scouts to do this, but I still ended up getting to most ruins before the AI, just by 1-2 turns in several cases, and went from an early 4-1 to a 19-5 artifact lead. The AI misplayed once, moving off an exploration ruin after researching antiquity sites, but otherwise was quite swift at getting to ruins.
I had another game where I produced my first explorers and the 8 turns delay caused it to go the other way 9-14 but the AI couldn’t get the last one.
Producing explorers seems infeasible, since you’d want to buy some with gold either way, and each buy delays the production by another several turns.
I’d rather the have culture govern the rate of explorers, either free ones with civics, or scaling their cost with cultural legacies.