Online Merriam Webster does a good job in introducing words on a daily basis. What it does is defining one word a day with definitions, example sentence(s), and backgrounds of the one word. Like today, it introduced a word, shoestring, and gave 2 definitions. One defnition is the literal meaning of shoestring, i.e. shoelace. The other definition is the metaphorical application of "shoestring," which means little sum of money. Applying the other definition, it gives an example sentence that says, For the first few years, Jillian and George ran the business on a shoestring. This sentence makes it easy to understand the metaphorical meaning of shoestring and makes it clear, showing how the word is used. Besides "unning the business on a shoestring," one could also say "taveling on a shoestring," meaning traveling on little sum of money, not having much to travel or one could also say "gardening on a shoestring," meaning gardening on little sum of money, showing gardening with little cost.
The metaphorical application of the word, "shoestring," came from the businessmen in earlier times. They usually wouldn't sell much or charge much for their shoe wares, and so they didn't earn much, either. This led to the connection of the metaphorical application of the literal meaning of "shoestring."
Therefore, in my opinion, the metaphorical application of "shoestring" is somewhat associated with little earnings, little money, not poor but not rich either, an in-between.