What is this lump?

What is this lump on my dog’s leg? She’s about 5-1/2 years old, and an American pitbull terrier. How much would it cost to get treated with and without insurance?

I don't think any vet could answer this question, even in person, the first time they looked at this lump.  At one end of the price range it might turn out to be a histiocytoma, which might disappear on its own accord within a few weeks and therefore might not need further treatment.  But it could turn out to be a bitten area of skin connected to pain from arthritis, which is ongoing and would require ongoing treatment, or a persistent histiocytoma that may be better removed, or Ringworm (a medicated wash), or something more eg a cancer which would need some degree of removal / further treatment.  The problem is that these could look quite similar to one another and while common things are common, it is hard for a vet to judge from the appearance alone what this lesion is.   They can do tests (usually starting with either the most likely or the one that is likely to be most urgent to know about) and ought to provide an estimate for that in advance if you ask them.  However, as clinicians joke, it is foolish to promise a cost of dealing with a skin lesion in advance because not every skin lesion has read the textbook.  Furthermore, we take questions from all over the world and most vet practises are private and make their own prices, so I am definitely in no position to do this.   I hope that something there helps.

Finally, you mentioned insurance - in the UK, the cost should be the same.  Usually you would pay a set amount and any costs further to that (up to another set amount) would be paid by your insurance provider.