For someone who equips 2 wizards, a cleric, and a druid for three runs a year, this is how I "valuate" scrolls. So YMMV, but this is the thinking behind what I think is a fair price for a scroll.
Damage scrolls scale up in price on a logarithmic scale.
Things that determine the price of a damage scroll (in my mind):
How much damage it can do: I start with a base of 2 GP/Pt of dmg
Negative modifiers:
Is there saving throw? Yes -1 GP/pt of dmg
Is there a slide? Yes -1 Gp/pt of dmg
(which means if they ever have a spell that has both a slide and a saving throw it's almost worthless)
Positive modifiers:
Can it affect multiple targets (this is what made chain lightning so valuable was the year we had the stirges it was the "I Win" button)? +1 GP/pt of dmg
Is it an "All?" - in this case that means classes like Paladins and Rangers can use them, which increases their usability. +1 GP/pt of dmg
Is it a "Divine" damage scroll - these are fairly new and much desired by divine casters who, knowing the sooner you kill the monster the less damage you have to heal, like to have these in abundance. +1 gp/pt of dmg (does not stack with the "All" +1 GP).
After that you apply the following.
Rarity Multiplier: Common x1, Uncommon x2, Rare x4-5 (caveat, if there is a lesser rarity spell that does the same, then use that: Ex: Fire Arrow and Scorching ray) (second caveat - there are times where I think a rare might be worth x5 - it really depends on how many of it I have in stock and how many I expect to burn through, for the examples below I will use x4, but don't be surprised if most merchants trade at x5)
Round to the nearest 5GP (just to simplify trading)
Next, there is the "Cheese-It" factor. If there is a scroll that can "solve" the problem (like the Chain Lightning vs. Stirges) or due to a known foe do extra damage (frostbolt vs. smoak) that can boost a price a variable percentage upto and at the convention.
Lastly, if can be used for a combo recipie that can often times double the value vs. comparable scrolls.
So lets see if the math works out
For "All" spells
Ball Lightning: 4pts, Slide, All, Common
So using the above formula (4x(2-1(slide)+1(All))x1 = 8, so round up to 10 gp.
Soundburst:3 pts,all, multi targets, All, uncommon.
(3x(2+1(All)+1(Multi))x2 = 24gp so round up to 25gp.
For Arcane spells
Common:
Shocking Grasp: 6pts, Slide, Common
(6x(2-1(slide)) x1 = 6gp (Round to 5)
Burning Hands: 5pt, Multi-target, Common
(5x(2+1(multi)) x1 = 15gp
MM: 4pt, common
(4x(2))x1 = 8gp (Round to 10)
Acid Spray: 5pt, Slide, Common
(5x(2-1(slide)) x1 = 5gp
Uncommon
Scorching Ray: 12pt, slide, uncommon
(12x(2-1)x2 = 24GP (Round to 25)
Shockbolt: 9pt, slide, uncommon
(9x(2-1)x2 = 18GP (Round to 20)
Frostbolt: 10pt, slide, uncommon
(10x(2-1)x2 = 20GP
Melf's Acid Arrow: 8pts, slide, uncommon
(8x(2-1)x2 = 16gp (Round to 20)
Rare
Chain lightning: 12pt, save, multi, rare
(12x(2-1(save)+1(multi)) x4 = 96gp (Round to 100)
Flame Arrow: 12pt, slide, rare (caveat - this is the same as scorching ray)
(12x(2x1)x2) = 24GP (round to 25)
Lightning Strike: 12pt, rare
(12x(2)x4 = 96GP (round to 100)
Divine:
Rare
Call Lightning: 8pt,Divine, rare
(8x(2+1)x4 = 96GP (round to 100)
So lets say Jeff creates a scroll version of Searing Light - something that could seriously help with dealing with Smoak
Searing Light: 10pt, Divine, Slide AC-15, Rare +8dmg vs. Undead
(10x(2+1(Divine)-1(Slide))x4 = 80GP, but because of its "Cheese-it factor" it would sell for at least another 20GP for 100GP each. And if the powers that be were cruel and made it part of the 4C it could easily double, if not quadruple, in value.
I hope the above makes sense.
I also use the same "valuation" criteria to determine what scrolls I give my casters to use - so for commons it mostly burning hands a couple MM to offset the fire immune crew.
For uncommons I only really use scorching rays and a soundburst on palidin and ranger if we need to saturate a room.
Then Rares - as many as I can get them to carry.
Our general strategy is use whats on your card before using scrolls, but knowing the value of healing is much greater than that of damage, I encourage my casters to burn the foe(s) down as fast as possible.
(BTW: I didn't forget the bard, but he is not my damage caster, he's my melee buffer - but I gear him a little like all the other classes when it comes to scrolls, a bit of this and a bit of that).
Victory Loves Preparation