I have 250 rounds of 30s-manufacture German brass ammo. What would the approximate value be?

Couple of bucks.  Maybe three-fiddy.

The biggest problem with most of that 30's manufactured german ammo is that they are total duds... not even a hangfire in the cases of ammo that I bought back in the early 90's. Dated 37, 38 and 39 with some 40's mixed in. I threw most of it away.

edit for splleingg

Threw it away

I love dud ammo! The bullet is still perfectly fine and you already have a pre-measured powder charge! Just add a new primed case and you have some perfect blasting ammo.

may want to try a ww2 collector page.  may hold more value for a collector.

I was young and dumb at the time and really had no knowledge of reloading and the costs involved... I bought them at a cost of pennies per round so it was no real financial impact, especially considering that actual factory new made rounds were over a buck per shot... Now of course, I would have saved the bullet and maybe the powder, just like I do nowadays with some M2 French made 30-06 from the 50's (which I am slowly taking the bullets out of, trashing the crappy brass, and using the powder as fertilizer). I did save one box of each of the ammo so I can have a historical marker of what their ammo looked like and the packaging of each. It was interesting to see the relative degradation of the ammo quality over the years, especially as the war started.

Yep, on WWII-vintage DM1 continuous link belts.

About 10 years ago I picked up close to 1000 rounds of WWII German ammo on M1919 belts for $150ish.

The steel case ammo has a failure rate around 75% while the brass stuff worked 99% of the time.  Various headstamps but all in the 1937-1942 range.

I pulled the bullets on 100 rounds of same headstamped steel ammo and most of the powder was covered in rust residue.  A few drops of oil to kill the primers and put the bullets back in and drilled a hole in the case for some nice dummy displays.