"That box set was less than $25 USD USD and came with 21 miniatures."
It came with 18 miniatures. Look it up. Even your own math confirms this.
"I also have over 150 Space Marines so I don't see a need to be forking over $160 USD USD+ for this when I already have more than what this set provides."
This is, of course, irrelevant to the overall value of the set. If the set were $5 USD and you don't need more marines you wouldn't be forking over $5 USD for them.
Plus the game is $150 USD, not $160. USD If you are paying more than $150 USD for it you are an idiot. GW won't charge you tax on it unless you live in their state and shipping is free on orders over $65. USD
The prices listed are not much different from the early 90s.
Even so your $50 USD in 1991 is worth $88.50 USD today. So the Eldar infantry figures in the box would cost about half that, or $44.25 USD
A land speeder, in 1988 cost 9.99 quid, or $17.57 USD. That would be $35.80 USD today.
So without the bikes, and we'll call those the balance you get for the four figures you aren't getting between the 14 you get in this set and the 18 you get in the Eldar set you claim was $25 USD, you still have $80 USD worth of today's money wrapped up in just the Eldar in the box. Minimum.
In 1988 you could get 30 simplistic marines for 9.95 quid or about $36 USD in today's money but it is difficult to compare those to the more modern figures which are superior sculpts. But lets say that is equivalent to the character, assault marines and tactical marines you get in this box.
That is then $116 USD without the dreadnought or the rules. I think the first dreadnought was released for the marines in 1995 but I might be wrong and it was 20 quid or $31.13 USD 1995 dollars.
So that is $47.65 USD in today's dollars.
Without the rules then and with some allowance for quality/quantity differences where exact matches can't be found, we are still looking at a $10 USD savings over the prices for the figures from the late 80's/early 90s with one model from the mid 90s.
The Harlequins are still being sold by GW in 1998. The reason they got rid of them was 3rd edition was released. Had they wanted to make them less powerful that would have been an opportunity to do so.
They probably got rid of them because the sculpts were a little static on a lot of the models and they wanted to redo them. I don't know exactly why they got rid of them.