LP has signed a billion things over the last 15 years. I think technically their signatures aren't worth much since the band has remained unchanged for the last 15 years also.
You won't find many signed skateboards, and the skateboards themselves are pretty rare too, so that should increase the value, but I can't really say how much you can ask for. It's more like how much is someone else willing to pay.
I think signatures have a much higher personal values, also because you are the only person that knows for sure that they are legit. A COA doesn't mean anything either, everyone can write one.
If you look at LPU auctions, some crazy people pay a ton of money for signed items, unless it's something really weird.
If you plan to sell it on ebay, do a buy now with a higher price but also with the option to submit offers. You probably have to be patient but maybe you get what you have in mind.
Again, you cant prove that the signatures are real and I think that's the main reason people why pay a lot more on lpu auctions or charity auctions, cause those are trustworthy sources.
How much were these back then? $60?