Shiplap how to do inexpensive ?

Cut plywood into the sizes you need. You could also look for used or damaged wood, or recycle some paneling.

cut plywood into the widths you want.. most DIY stores will cut it for you (usually 25 cents per cut)..

or.. paint the wall a shade of white, and put painters tape horizontally across the wall every couple inches (to mimic the width of shiplap).. and carefully paint dark grey lines (or use permanent marker)

If you are doing shiplap in a relatively small area, you can 'fake' it with creative painting.

bead board!!! you can get the paneled bead board and put it sideways and its like 20 bucks a panel and it is quick, easy and you can paint it or even get wood colored bead board.

my bead board ship lap in my tiny laundry room!

I did the same as suggested above, had Home Depot cut strips. I actually used 1/8" type plywood, it's almost like a ballsa wood, very thin. I nailed it up on the walls, stained it and painted it white and then sanded it in miscellaneous areas so the stain would show. My picture below doesn't actually make it look as good as it does, its much more subtle.