How much rad resist is needed for The Glowing Sea?
Spoilers ahead
If you finished the quest The Secret of Cabot House, you acquired a couple doses of Mysterious Serum.
Instead of increasing your rad resistance, that serum removes 10 rads per seconds during 3600 seconds (an hour). Since most areas in the Glowing Sea inflicts about 10 rads per second, you can cruise around without any effects. A single dose will last long enough for you to do the quest and explore the area.
I popped like 2 rad-x and was fine. I think I used about 3 rad away while navigating, but it wasn't anywhere as near as bad as the NPC's lead me to believe.
However, there are some areas you won't want to go walking about in, as they have far higher rad ratings.
You don't need a tremendous amount at any one time, unless you stray into certain high radiation areas.
There are a few ways to do it.
Consumables
On my last playthrough, I made it from the Edge of the Glowing Sea to Virgil's Cave on two RadAways and two Rad-X.
On the way, I discovered the Capsized Factory, the Crater of Atom, and the Decrepit Factory. I was wearing 25 rad resistance from an Assault Gas Mask and the Vault Suit. This completely took care of the radiation. I arrived at the cave with zero rads. Note that I was in a hurry. I did not fight a single creature and ran as much as I could.
Watch out for areas of ground that are glowing, or contain liquid. The radiation can be extremely high in these areas, and can overcome the rad resist from a single Rad-X and some clothes.
Rad-X does stack, so you can take several if you really need to enter a high-radiation area.
You could also use the Mysterious Serum if you have it. This gives you -10 rads/sec for six minutes.
Hazmat
A Hazmat Suit or similar outfit provides 1000 rad resist, which is plenty. You won't even need any Rad-X. You'll be extremely vulnerable in a fight though, so you'll want to hustle to your destination and avoid engagements.
Power Armor
Power armor provides as much or more rad resist than a hazard suit, but also gives you great protection in a fight. If you're going to be extensively exploring the area or fighting enemies, this is the way to go.
I can consistently make it from the Edge of the Glowing Sea to Virgil's cave on less than a single Fusion Core charge.
Cost Comparison
Rad-X has a value of 40. RadAway has a value of 80. Fusion Cores have a value of 200.
So, if two Rad-X and two RadAway are roughly equivalent to one Fusion Core, that's 240 caps for the chems versus 200 caps for the core. The chems are actually more expensive.
Of course, a hazard suit doesn't cost you anything to wear it, so that would be the most cost effective solution, assuming you can do what you need to do with no damage resistance.