As a few others have said, there is no set % that you can put over "book value" for a graded card. High grade cards like BGS 9.5 and 10 and PSA 10's generally command much larger premium than do grades that are lower. For most modern cards (2000 and after) if you don't get at least a 9 you are probably not increasing the value of your card by grading it. Even a 9 would likely gain only a small premium over BV as most modern sets have good production and are not particularly condition sensitive.
The value of the card is what somebody is willing to pay for it. The reason the Montana card commands such a premium is because it is rare to find the card in that condition. As you stated, there are 6,111 BGS graded 1981 Montana's and only 65 BGS 9.5's. This means those 65 BGS's represent the top 1% of BGS graded examples of those cards and that 99% (or 6046) of the other graded cards are considered to be in worse condition than the 9.5.
In comparison, your Luck is one of 10 other BGS 9.5's with one other 10 and one other 9. This means that there is only one card graded in lower condition than yours and that a 9.5 falls in the top 91% of graded examples of that card. That means while there are only 10 of your cards compared to 65 of the Montana's, the Montana is significantly harder to find in Gem Mint condition. As any 9.5 Montana represents the top 1% of cards in that grade, while the modern Luck represents 91% of the cards graded so far. That is why the Luck card will not command the same premium as the Montana.
Typically the older the card, the higher the premium placed on higher grade cards. When I want to know what a card is worth I generally look at Ebay sold prices for an idea of what a card is worth as Book Value tends to over state the value of ungraded cards (in most cases) and with graded cards can often over value or under value the card depending on the graded cards relative rarity.
The value of the card is what somebody is willing to pay for it. The reason the Montana card commands such a premium is because it is rare to find the card in that condition. As you stated, there are 6,111 BGS graded 1981 Montana's and only 65 BGS 9.5's. This means those 65 BGS's represent the top 1% of BGS graded examples of those cards and that 99% (or 6046) of the other graded cards are considered to be in worse condition than the 9.5.
In comparison, your Luck is one of 10 other BGS 9.5's with one other 10 and one other 9. This means that there is only one card graded in lower condition than yours and that a 9.5 falls in the top 91% of graded examples of that card. That means while there are only 10 of your cards compared to 65 of the Montana's, the Montana is significantly harder to find in Gem Mint condition. As any 9.5 Montana represents the top 1% of cards in that grade, while the modern Luck represents 91% of the cards graded so far. That is why the Luck card will not command the same premium as the Montana.
Typically the older the card, the higher the premium placed on higher grade cards. When I want to know what a card is worth I generally look at Ebay sold prices for an idea of what a card is worth as Book Value tends to over state the value of ungraded cards (in most cases) and with graded cards can often over value or under value the card depending on the graded cards relative rarity.