How do you figure between grade prices?

I don't have access to the grey or blue sheets, and usually just use the price guides in coin magazines. Most of these mags only give prices for the major grades (VF20, XF40, AU50, ... MS65). Is there any rule of thumb I can use to figure out the price of a coin grading in between?
In this particular instance, I'm trying to price an 1884-S Morgan in XF45. The magazine I'm looking at says $41.50 in XF40 and $280 in AU50.
The way I see it, I can:
1) Figure since it's still an XF grade it should be closer to that price - $60
2) Take the half way point between grades - $160
3) Figure in the rate of increase between grades and interpolate - $100
4) Say PCGS is way better than other coins and worth full AU money - $280
I'm thinking 3 sounds the most realistic.
In this particular instance, I'm trying to price an 1884-S Morgan in XF45. The magazine I'm looking at says $41.50 in XF40 and $280 in AU50.
The way I see it, I can:
1) Figure since it's still an XF grade it should be closer to that price - $60
2) Take the half way point between grades - $160
3) Figure in the rate of increase between grades and interpolate - $100
4) Say PCGS is way better than other coins and worth full AU money - $280
I'm thinking 3 sounds the most realistic.
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In some series the price jumps significantly even in circulated grades. An example of one I was discussing the other day was the 21-s WLH. In VF the coin lists at 500.00 , in Xf the coin lists at 3500.00. Where and how do you price a Vf-30 or Vf-35 coin to be fair to the buyer and also the dealer. Depending on the service, the looks of the coin, I have seen a vf-30 sell for anywhere from 700.00 up to and over 3k.
The biggest problem with trying to determine a value of a coin that falls in this range is usually when there is a potential deal on the table. And whetehr you are looking to buy, or wanting to sell your coin?
If it's an auction coin, people who bid on it are going to decide whether it has a shot at the next grade. The better shot they think it has at an upgrade, the closer the coin will sell (with commission) to the FH 6 market price. OTOH, if people think it's a really nice FH MS 65 & will not upgrade, the coin may go for sheet plus 20% (with commission).
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Since we are talking about SLQ. There is a PCGS VF35 1927-S SLQ in the last Heritage auction. VF-20 is about $120, EF-40 is about $850. The coin was sold over $1500. It is a well under-graded coin. At least several folks in the auction was thinking about the same thing. Of course, some one wants to pay one grade lower money; someone will pay moon money to get it. How much you will pay it if you want it. At least, I can tell you that in that particular auction, you won't get it if you want to pay sheet money.
For instanace, a 1913s barber quarter, in VG with 5 letters in liberty instead of 3, how much of a premium does that rate above a VG price?