There is speculation and investing in a sure thing.
Modern books are tricky. I am a huge Spidey fan but have not read a Spidey comic that has come out in the past 20 years. Why? It is all rehash of older original material, in some way. Granted, there are a few newer storylines that are interesting but not as much as Silver or Bronze Age Spider-Man for me.
Red Goblin, to me, looked absolutely hideous. Poorly drawn, ugly covers. As a collector, I have zero desire for that book. Maybe some day, he appears in a show or movie ans we see his first appearance bump up a bit, sure. That is commonplace now with comics. But if you want to put money somewhere smarter, invest in original characters. Like Green Goblin of Hobgoblin. Both are great characters, albeit one is an offshoot of the other, but was done very well. They will always be collectible and stand a better chance of being more valuable. More of a sure thing.
Another thing to consider is do people care? When Red Goblin first came out, yes people bought that book. But I feel now nobody cares about him because it was a ho hum character in a ho hum story. What people do care about is something like Immortal Hulk. Every issues of that arc has been great, artwork is also very very good. It is a series that has staying power because it is well done and because it could also someday be adapted to the silver/small screen. More of a sure thing there as well.
But hey, if you love Red Goblin and believe in it, go for it. You could be the guy 20-25 years from now with 100 copies all worth $100-200 bucks each and be laughing. You could also be the guy with 100 copies worth maybe $200 bucks total. Invest in sure things, not speculation unless you are prepared to have some misses when speculating.