Good that you are saving money for the vet, better get pet insurance.
Lumps and bumps can be benign or malignant, just by looking at it or feeling it no one can tell.
Some are painful, most are not. Saying that most dogs can be quite stoic and do not show pain.
A vet can do a fine needle aspiration and extract a few cells and check them under the microscope. That is probably the cheapest option.
Pros: cheap, immediate results
Cons: it can happen that there is a mixture of benign and malignant cells and the aspirate only got the benign ones. Also, in some cancers malignant cells can get into the needle canal while extracting the needle and then spread (I think this is not a concern for this lump though)
Another option would be to have the vet do a biopsy. That means that the vet takes a sample of the lump and sends it to a lab.
Pros: better chance to get a good combination of cells, lab can do more specific tests than the vet
Cons: Dog has to go under anesthesia and if the lump is malignant the dog has to have surgery again
The last option is to have the lump surgically removed.
Pros: with good margins the outlook is usually very good, if it has not spread yet
Cons: expensive, dog needs anesthesia.
If it was my dog I would probably go for a fine needle aspiration first and then decide the next step.
Dagmar