And if you do get them cast think of how the poor machinist will hold them, may be worth adding a lug each end which can be used to clamp the part with and then machine that off at end of job.
Are the pedal holes R/H and L/H threads? Is there also a hole for a cotter and has the bottom bracket worn due to loose pedals overtime in which case a 9/16" may be a bit loose. All things to think about.
That dashes my first suggestion which was to just go out and buy new arms.
Honestly that machine shop quote wasnt that bad. Back in the day when parts for injection mold bases where quoted we paid that much for something much smaller and less complex.
You might want to contact one of the youtube machinist like NYCNC to see if they can take on the project. It is an interesting piece to teach some CNC techniques. It will still cost a bit but you might be able to make a deal in exchange for video time.
To reproduce this in a manual machine shop would likely cost even more as the part would be setup intensive.
As for casting i wouldn't go the last wax method and instead make a pattern out of wood or even metal. It is still a bit of work but you could likely do most of the pattern making on a wood lathe.