One (1) frag grenade costs 276 caps.
One (1) frag grenade is made from TNT, Hexogen, Fuse, Oil Can and some scrap metal (metal parts x5). It's one of the few items that need 2 rare(-ish) items (tnt, hexogen) and 2 get-for-caps items (fuse, oil) to craft.
A frag grenade is comparable with a 40mm grenade, but the later is bought for 10 caps each (box with 10 for 100 caps).
One (1) plasma grenade costs 276 caps ( or 95caps for 50MFC + fuse if you have a sh*t load of electronic parts).
A plasma grenade is not exactly comparable with a Plasma Cannon shot, but cost-wise a Plasma Cannon shot for 10MFC is worth 9 caps.
One (1) incendiary grenade costs 138 caps at the shop or 80 caps if you craft it yourself (fuel mk2 x5=20caps, chemical components=10caps, fuse=50 caps, metal parts+alloys= free).
An incendiary grenade is comparable with a Incinerator shot and a 10x napalm fuel box is worth 50 caps.
I'm not even asking to boost the grenades, just make them cheaper.