If you fill the cores by pouring in expanded perlite, you will increase the R-value from about R-1.6 to R-5.9 (assuming 8" 2-core medium-weight blocks). That R-value is for the whole wall, not just the perlite (so it includes the loss from the webbing in the blocks).
Call your local masonry supply companies to get availability/price on perlite insulation. It's sometimes sold in 4 cubic foot bags.
Below I've tried to estimate how much perlite you'd need so you can figure your cost. Skip to the examples at the bottom for the short answer.
The general formula for 8" 2-core blocks is
volume_in_cubic_feet = perimeter_in_feet x wall_height_in_feet x 0.26
Don't forget to subtract the width of doors from your perimeter.
The perimeter of your 24' x 40' building is
24+40+24+40-(door_widths)
or
128' - (door widths).
Your 16" high walls are 1.33' tall.
The surface area of your wall in square feet is
1.33' x (128' - door_widths_in_feet)
The volume in cubic feet of the cores of your 8" 2-core blocks is approximately
wall_surface_area x 0.26
or
1.33' x (128' - door_widths_in_feet) x 0.26
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As an example, if you have a 12' garage door and a 3' wide walk-in
door, and the perimeter of your building is 128', then the volume of
the cores for your blocks is
1.33' x (128' - 15') x 0.26 = 39 cubic feet of perlite = 10 four-cu-ft bags.
==========
As another example, if you have two 12' garage doors and a 3' wide walk-in
door, and the perimeter of your building is 128', then the volume of
the cores for your blocks is
1.33' x (128' - 27') x 0.26 = 35 cubic feet of perlite = 9 four-cu-ft bags.
