my beman carbon alum inserts weigh in at 22 grains. or as crandog mentioned, there are also pdp screw-in weights for the rear of most any insert.
if yer looking to mess with mass arrow weight and foc, consider what i do - weigh yer current alum insert, or an aftermarket brass insert, as a base front end starting weight, then add in the right combination of adapter and point weight to add up to the target front end package.
alum adapters run around 30-40 grains, steel adapters come in 75, 100 and 125 grains. most any glue-in point types (broadhead, field, judo) start at 100 and can go upwards of 200 grains. that's a very versatile assortment of point types and weights. use slow set epoxy to glue in inserts to shafts, and points to adapters.
using one type of shafting, beman bowhunter, i use a 100gr brass insert + 125gr steel insert + 125 point for a 585gr arrow (29% foc) for my 55# longbow. i use the same shaft, but with a 22gr alum insert + 100gr steel insert + 125 point for a 480gr (25% foc) arrow for my 46# longbow.
gotta love the versatility of tube shafts + inserts and adapters! :cool: