Pro tip, you can cut laminated tempered glass... think a windshield... just not advised because metal on tempered glass may cause explosions.
Learned this little trick when I did my first chop top on a shoe box Chevy sedan. 3 windshields later and a few stitches I got it cut, but won’t be jumping at the chance to do it again.
As far as cost is concerned, what’s cheaper? Water remediation or buying proper glass? Odds are it’s going to be the glass hand over fist.
To echo Hendre, buy the iron glass, you will be happier with the clarity over standard plate glass. As plate glass may have imperfections in an undesirable place. Plus there is going to be a changing load onto the glass during WC, therefore it’s better to build with this factor taken into account instead of just thinking of initial cash outlay.