It depends on the part. The first things I did for my army were just a few conversions by taking existing figures (Warhammer preist, plastic tank commanders from LR kits) and sculpting furry hats and equipment.
For converting my cadians eventually into vostroyans, I am going to build/kitbash/sculpt a vostroyan long lasgun. It will take some work, but once it's done, I'd rather cast it than make two dozen. I could make 3, build a sprue and then cast it a small number of times.
I'm also working on a sprue of heads for easy conversions of other figures.
If it's a part based from readily available pieces, building it is easier, although not as cost effective. I built jump pack wings for my Blood Angels years ago using power armor packs and eldar wings. I had a ton of the parts, so I built 3 dozen sets. But if it was something where I had to sculpt the wings, I would work to sculpt two or three poses and then cast the heck out of them. Saves time and money.
John M.
(I, in no way, condone or support the casting of existing GW parts, as that would be illegal in most places, even if for "personal use". I do support and encourage sculpting and casting your own pieces and parts.)