Of course it’s more reliable to look them up, but the point of doing a Fermi estimate here is to figure out what else you might be mistaken about. For example, if you do a Fermi estimate about how much a rocket costs and it turns out to be really inaccurate, you might learn that that’s because you have a really inaccurate picture of how much engineers get paid, or how many engineers it takes to build a rocket, or how much the materials for something like a rocket cost, or…
Of course it’s more reliable to look them up, but the point of doing a Fermi estimate here is to figure out what else you might be mistaken about. For example, if you do a Fermi estimate about how much a rocket costs and it turns out to be really inaccurate, you might learn that that’s because you have a really inaccurate picture of how much engineers get paid, or how many engineers it takes to build a rocket, or how much the materials for something like a rocket cost, or…