Looking at typical prices, fine tungsten powder is only about $15/kg (USD), which corresponds to less than $0.001/grain, which means it wouldn't add much to the material expense of manufacturing bullets (as a filler, not an alloy). A 125gr bullet 50% tungsten by weight would add just six cents to its material cost and increase its density by 35%.
Since tungsten is more dense than lead (19.3g/cc vs 11.34g/cc), mixing it into lead would allow for a lower bullet volume at a given sectional density. This means bullets could be given a deeper, narrower ogival shape without sacrificing sectional density nor increasing their mass nor increasing their length, for an improved ballistic coefficient.
The main problems I can see are (1) the (slight) toxicity of tungsten might put off manufacturers or buyers and (2) getting the tungsten to distribute evenly in the lead might be problematic, and bullets with uneven weight distributions don't spin-stabilize well.
Edited by TTK Ciar, 16 April 2018 - 1225 PM.