Dude. Thats a lot of work to remove ALL that old mulch. I would charge them your hourly rate to remove the old mu;lch until you get good at estimating your time right. I would also up-sell installing landscape fabric once all the mulch is out. It takes time to lay fabric down, but its worth it.(Cut up metal hangers for pins to hold down fabric, Bend them like a horseshoe)
Taking out old mulch and dump on site, lay fabric and install 12 yrds new mulch is easily a $1400 job. Again thats a ballpark number. I dont know the layout.