Think of the biggest case you could possibly need. Now double that. (Honestly, I've never met anyone who regretted buying too much mini storage capacity.)
Are your bases magnetized though? I made a carrying case that doubles as a cheapo display case out of a Hefty under-the-bed storage and discount store cookie trays:
Cookie trays are secured to the lid with nuts and bolts. Cost to make one of these things is roughly $16 each case ($7 container, + 2x$3 cookie trays + $2 PVC spacers + $1 nuts, bolts, rubber washers).
Look ma! No hands!
I am currently in the process of repainting and rebasing some of my earliest minis to work with these things.