You've answered your own question.
Actually, spotting dyes are very inexpensive. I bought a five-pack of Spottone more than 30 years ago that is still going strong. The key is in how you use them. If you work with wet dye, it will go quickly (and spotting will be more erratic). But if you work with dried dye, it will last forever and you will have far better control over the spotting process.
The trick that I was taught (by David Vestal) is to use a palette. Vestal used a china saucer that I picked up at a garage sale - I opted for a cheap white plastic makeup kit. Place a couple of drops of the dye on the palette and allow it to dry thoroughly. Then, moisten a brush with distilled water that has a drop of PhotoFlo added to break down surface tension. Smear a bit of the dried dye across the palette to create a range of shades of the dye. Rinse out the brush.
After selecting a spot to work on, using a damp brush, pick up a bit of dye of a shade slightly lighter than the area surrounding the spot. Apply the dye to the print as a series of dots - don't try to paint the dye on the print. Carefully and slowly fill in the spot with dots of dye. Keep in mind that the objective should be to reduce the local contrast to the point where the spot no longer stands out against the background. If you use a shade of dye that is slightly lighter in color than the area surrounding the spot, and you find that the spot is still visible, you can always add more dye. But if you use a shade of dye that is darker that the surround, you take the risk that the spotted area will be darker than the surround, and removing excess dye is very difficult.
Take your time, work under bright light, and use a magnifier of some sort. I use drugstore reading glasses in a strength that is a couple of diopters stronger than my reading prescription.
It helps if the light is directional. In that case, the light will cause a shadow of the brush to fall on the surface of the print. If you carefully bring together the tip of the brush, the tip of the shadow of the brush, and the spot, you will get the brush exactly where you want it to be on the print. If you don't have that shadow, you will have to rely on the depth perception of your eyes to get the brush to the right spot and that's more difficult.
After you are done, put aside the palette with the dried dye for the next time.