Those are crazy prices for cast. Locally, they cost around $120 for 1/4 and $170 for 3/8.<a href=showthread.php?s=&postid=8093152#post8093152 target=_blank>Originally posted</a> by bower23
I was just looking at that myself, for a sump. The prices I got were $184.00 for 1/4 inch and $436.62 for 3/8, these prices are for a 4x8 sheet. I think you would be ok with 1/4 but if you went any bigger I would go with 3/8. Sumps are a little different because the divider sections also help with support.
<a href=showthread.php?s=&postid=8142622#post8142622 target=_blank>Originally posted</a> by kgross
With 1/4 and no bracing you will get a little bit of bowing but I would guess that it will be less than 1/8 inch.
But it should be plunty strong enough as long as you have good joints.
Kim