Rules wise I found it simpler to graft on the rules bits from D20 Modern spiced with a spot of the new pathfinder . This set up is compatible with 3x and the rules while far from perfect work well enough for me.
Costing firearms was a bit trickier
Typically I assume flintlocks cost about 3x what an equivalent bow does with ammo running roughly 1sp per point of maximum damage the weapon can do. Lead balls can be had anywhere for about a copper..
Modern Firearms are a but pricey, costing around 1000 for a handgun and 5000 for a long gun, based on weight and the cost of the casting of 7th level spell plus markup.
Ammunition is also crazy expensive costing costing about 15GP per handgun round and 30GP per shotshell or Rifle round. This premium keeps the number of them in circulation small. The small number of casters capable of handling 7th level spells means that the number of military units with these weapons is small.
As for energy weapons, they exist but are rare and cannot be bought.
As above I tend to use modified D20 Moder/Pathfinder rules.
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
13 hours ago