Brock, I recall PPTSS #27 (1st Miller DD) always being more expensive than surrounding issues, likewise #85 (Early Hobgoblin appearance/1st time he obtains superhuman powers). Also, PPTSS #90 (1st Spidey black costume in this title) has seen a significant bump over the past 5 years or so.
Overall, PPTSS is a decent read with solid, if somewhat unspectacular artwork, but I wouldn't consider it a substitute for ASM, no matter how pricey the parent title becomes.
To other posters thinking that the title is undervalued, it most likely is. But us old timers have been saying that for decades yet prices are still where they are today. Let's face it, PPTSS #1 is a 47 year old book, the start of a long running popular title, and it lists for under $100 in Overstreet today.
RE: 1st Cloak and Dagger, they were hot for a micro-second, had subsequent appearances in a relatively short time in PPTSS (never in ASM, and only once in MTU (an annual where they co-starred with other heroes)), and landed their own short-lived ongoing series. Oh, I almost forgot they co-starred briefly in the re-launched Strange Tales (split book with Dr. Strange). Made a few appearances in New Mutants (Sienkiewicz run) and that's about it, I think.