Second, in order for the cost to be worth it, this coin would need to be worth in excess of $100 or perhaps $150.
I disagree here and am responding in a general sense. $30 is generally the lowest coin cost that you can really make any money on and even then it isn't much, but you could easily make a good profit on $50 dollar moderns. Moderns are only $16 to grade at PCGS
so its a lower fee than classics. The cost of getting the modern you want will play a role as well, but anything you got at face value roll hunting or change will absolutely be in the nice profit range for $40-50+ coins.
The real question from a money standpoint that should be asked is will the grading increase the value but the cost of grading or more. If I spend $4 on a modern and can have it graded for $16 (17-18 accounting for shipping and everything in a good sized submission) but I know that it will grade out to be a 50 dollar coin you've done well. Try selling that same coin for 50 raw and people will make fun of you and call you a scammer.
Point being instead of focusing on a floor value the focus should be over the overall impact for that particular situation
I know they give some free grading with the 2nd membership level. It costs more but I think the free grading value cancels out the cost of the membership for the most part, where the lowest membership tier you are just giving them money and paying the grading fees as well.
if someone is interested in joining the Platinum membership for $250 is by FAR the best one to pick. You get the 8 grading vouchers which if you use for the regular gold shield tier their value is $320. So not only does it cancel out the membership fee but it actually saves you money when you use them properly.
You also get a dri-fit Nike PCGS
shirt with that that you can sell for $15-20 on eBay if you don't want it to get even more of a savings. I would honestly rejoin every submission if they would let me.