When Purchasing Slabbed Coins Price?

A lot of variables here. What slab is it in? PCGS, NCC, ANACS, ICG? In that order, highest to lowest premiums are, USUALLY, what said coin will trade for. Where are you buying said coin? Major auction house, ebay, LCS, coin show? In that order, highest to lowest are, USUALLY, what said coin will trade for.
PCGS, Numismedia, Greysheet, all great price guides, but sometimes they are high and sometimes they are low, depending on that coin and the current market.

That said. My approach. Ignore the slab, it's a good reference but not the end all. If the coin is solid for the grade, stronger than average strike, possibly earlier die state, strong luster, no major distracting hits; it will probably sell for a premium, possibly over said price guides. If the opposite, then a lower price.

When I spot a coin I like, I check Greysheet, recently sold auction prices, sold ebay listings, and make a call based on those prices in combination with the overall appeal of the coin. Sometimes it's below GS, sometimes it's over retail.

And sometimes I just see a coin I can't live without and toss all that out the window!

I usually take buyers premium into account, somewhat. But never take shipping into account. Shipping is usually the cost of my gas money or less, driving around trying to tack it down anyways.

Study the market, and most importantly understand the coin your buying! In reality, slabbed examples will typically sell for more than raw, but try to treat every coin as if it were raw.

I would check ebay sold values as well as Heritage Auctions sold values.
John1