They have: Exterior 4mm Okoume $34.02
: Lloyds Certified Okoume $62.00
: 4mm Sapele $80.71
: 5mm Mariante (SP?) $33.07

I think that last one would be meranti. It is used the same as Lauan, and sometimes sold as lauan. I've seen "1/4 inch Lauan" advertised at the store -- and stenciled to the side of the stack is the word "meranti", and 5.5 mm. So much for truth in advertising. You may see more meranti in the stores as there are political protests about over-logging of Lauan in some countries.

It used to be that a lot of species, including these two, were sold as "philippine mahogany".

(see: http://www.forneyauctions.com/wood/mahog_phl.htm for a longer blurb on this. And then an even longer blurb with LOTS of info is at:
http://www.asiarainhardwoods.com/balau.html )

They sell 1/8th inch sheets of that stuff (meranti) near me for floor underlayments. The front side is great, but the back may have some rough spots. Color varies from sheet to sheet, too. but they want just under $8 a sheet for it, so I can afford to buy twice as much and cut away the bad areas.

Make a few more phone calls and keep looking for better prices. Unless these prices are stated in Canadian dollars, or the panels are certified and rated to either bs1088 or bs6566, they seem a bit high -- which I am surprised at for Maine.

That 5 mm "Mariante" as you call it, would be a nominal 1/4 inch luan or meranti at any lumberyard, assunming this is NOT a marine rated panel. In the midwest it should be under US $12 a sheet. I had thought that marine plywood would be cheaper in areas (like Maine) which are closer to traditional boat building sites.

If what you've listed is indeed marine rated, then forget my comments -- although I'm seeing Lloyds certified bs1088 occume in Chicago for $34 a sheet, and the dealer said he had previously stocked the same stuff, but uncertified, for a few dollars less. From this I gather that the difference between certified and uncertified is not as great as your dealer would like to make it.

Hope this helps

PGJ