Depends on how dense the board is. Infill is a choice when printing, so more plastic means more cost. I get my filament from $10 to $20 kg. Depends on sales and colors and such. These prints cost me $2 ish.
I wouldn't do that in the slicer, it's too hard to align them. I don't know about any alignment tools in slicers. I could do that in Fusion360, however.
Got them for free, other wise I wouldn't. They're also only 3 gallons, so a lot easier to handle. Haven't tried them yet, but a porter is going in this weekend.
Yep. That helped!