This is not uncommon when attempting to quantify DNA after agarose gel purification. Hence I quit doing in decades ago. Instead, take an aliquot of your purified digested plasmid and run it in a gel (with EtBr) along with a DNA ladder with known quantities for the bands (i.e. NEB markers). You can compare the intensity of your digested plasmid band to the ladder bands and get a pretty close approximation of the concentration of your DNA.

Tom Masi Thank you! I'm new in the experimental area, that's why..

No problem! A piece of advice I learned a long time ago is your eyes do not lie to you, a spectrophotometer will.

Good Luck!