I've never seen an older Series I guitar (NOT bass) go for much over $3K on eBay. I bought my 1976 Series I medium scale guitar in 2006 and my 1977 Series I long scale 12 string in 2007. Both came off of eBay, both were in excellent condition with cases. The '76 didn't have a power supply and sold in the $2-2.5 range. The 12 string had a blue box power supply and sold in the $2.5-3K range.
The most recent Series guitar that sold on eBay was 74-53, a beautiful long scale guitar with a Fenderish six-in-line peghead that sold about three weeks ago. It had some holes drilled into it from a previous Roland Synth pickup, a headstock repair and a DIY power supply and case. It went for $2730. As previously noted the Santana guitar is probably not #28 and, more likely, closer to #53's vintage. I agree that if the guitar came with some provenance of the Santana connection it would increase the value, but without validation it's just a cool story - like the one that I received when I bought #10 that it was made for Jerry Garcia. If anyone wants to pay $4,500 for an older Series I guitar, I'd sell mine.
Bill, tgo