One possible option would be these 8x8 pixel displays.
I got that from eBay for $8.50 (for the 4 modules).
They come pre-assembled, this is the back:
Notice the data direction. You can connect them together like this:
That one (3 lots of 4 modules) is 25.4 cm long (10 inches) and 3.2 cm high (about 1.25 inches).
That's 96 pixels (8 x 4 x 3) in 10 inches, which is a lot less than 40 DPI, but really, if you want to read them from 15 feet away, that is probably OK.
Note: These were single-colour only.
If you want 40 DPI x 6 inches that means you must want 240 pixels horizontally and 120 pixels vertically. That would be
240 / 8 modules horizontally (30 modules) and
120 / 8 vertically (15 modules). That's a total of 450 modules. Since they come in lots of four that would be 116 in total (rounded up). 116 x $8.50 = $US 986.
I don't know if that is what you have in mind by "cost cutting" but that is one option.
You would need to assemble them yourself, connect them together, supply power, and have a suitable driver. I have some code that drives those boards, although I certainly have not tested it with 450 of them at once.
As you can see those boards come with mounting holes on the back, and are designed to be physically adjacent to each other. Mounting 450 of them could be quite tedious. The LED modules on the front pull off to give access to the mounting holes, but it would be a long job.
You can get larger modules, as mentioned in another reply, I expect they would be more expensive, but you would of course get more pixels.
Example: 64x32 RGB LED Matrix - 4mm pitch. That one is 64x32 pixels for around $US 80.
You would probably need 4 of those horizontally (256 pixels) and 4 vertically (128 pixels). Thus, a total of 16 which would cost you around $US 1280. However for that at least you get RGB. Note that the Adafruit page mentions you cannot drive that many pixels with a Uno.