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Is there a $ threshold at which one should start looking for certs?
There's a myriad of different facets involved in answering that question but here's a few thoughts off the top of my head (and hopefully others will chime in as well):
One's own comfort level is in play here and this includes things such as price and one's confidence in their own ability. Fifty bucks could be a huge investment for one collector while to another it could be 'pocket change.' A cert for any size investment does add a level of comfort towards a purchase, regardless of one's budget size. Certs (being an 'opinion' arrived at by experts) add to the value of a stamp, but for lower cost stamps, they are an expensive proposition (catalogue value vs cost of cert).
Studying the stamps and the catalogues, specialist society memberships, and if you're fortunate enough to have a pal who is an expert who can mentor you, can build one's self-confidence to the point where one's personal price trigger point could be raised before the need for a cert is felt. This way you will learn about things such as which stamps are more commonly found to be, for example, regummed (in my limited experience, the Columbians in MNH would fall handily into this category). If in doubt, get a cert (there are few things in the hobby more disheartening than finding out one's pride and joy, perched on an album page for years, turns out to have been reperfed or has hidden faults when going to sell and the buyer puts the stamp on extension). Again, its all about one's hard-earned knowledge and comfort level.
Clearly, the desirability for a cert increases in direct proportion with catalogue value, even for advanced collectors who are sure of what they have but desire the safety margin a cert provides, especially for future sale.
Sorry if this a rambling post, but I'm typing in between bidding in the currently running Golden Oak live auction on SAN

The bottom line in my answer to your question...It depends...