Weapon RP costs analysis
Warning: Charts and math ahead
This chart shows the total research points invested to get to the technology required for each damage level of weapon.

Ignoring the weapon size differences for a moment, there are a some oddities to notice.
- The research cost for guns is extrordinarily high; to purchase a Damage-4 Spike driver costs 33,575 RP; that is twice the research cost of a Damage-4 Phaser-IV (16,725 RP). By the time I research Guns-5 (Spike Driver-IV) for 49,875 RP, I could have researched Damage-22 Doom Rays for 50,025 RP.
- The beam-6 weapons costs much more than the beam-7, beam-8, and beam-12 weapons.
- There is no beam weapon around Beam-15, but there is a technology called Subspace Anihilator.
Factoring in the size of weapons is a bit trickier. Straight multiplication by the size of the componant gives the following, which still shows guns as too expensive to buy:
Edit: Changed formula to RP/(size*sizemod) as recommended below, to include differences in sizemod.
and that doesn't even factor in the sizemod values.