IMO, it depends...
There are some extras that are justifiable. This might be because the SP doesn't want to do them too often, such as anal, impact play (especially if hard/marking) or rough sex, but otherwise is OK and does them well. Alternatively, she might allow for preparation time and/or clean-up time, such as anal (again), wearing latex, complicated costumes or sploshing.
Then there are extras where the SP doesn't want or like to do them, but for extra money will allow them, although delivery is often poor as the reluctance comes across. Examples might be CIM, fingering, deepthroat or watersports.
Finally, there are those SPs just taking the piss. Kissing, receiving oral, 69, uniforms, etc., etc. Definitely to be avoided as a negative experience is on the cards.
Generally, each case needs to be assessed separately. What is the overall price? Are there one or two OK extras, or a long list? How does the profile come across overall? What do reviews on here say?