First, I want to advise there is a difference between "expunging a record" and "sealing a record". You can have a record "expunged" if you are found not guilty at trial. Otherwise, and this applies to a "deferred" adjudication, you file what is called a Petition for Non-Disclosure. This is same thing as sealing the record. Think of it this way, if we expunge, at least in theory, we destroy all the record of it ever taking place. A non-disclosure or sealing of the record means that the arrest (which is the part that shows on your record now) cannot be disclosed to any person or entity and it becomes a violation of a court order if "Public Data", "People Search", a Court Clerk, Officer of the Law, etc. tells anyone you were arrested for the offense that is ordered non-disclosed.
Ultimately, it is semantics. Both achieve the goal you are looking to accomplish - no one but you will ever know, and you are not telling anyone. Whether the information in question is stored and locked away from prying eyes (Non-Dsiclosure) or permanently deleted (Expunged) is a matter than could fill volumes and in the end, it would not change your life one way or the other - with one exception that I know - if you want to work for the FBI or the CIA or some other alphabet organization that has access to stuff no one has access to, you may have to explain it during a job interview, then again, you may not because the non-dsiclousre covers the TCIC and NCIC (Texas and National Criminal Information Computer(s) respectively) as well.
Finally, to try to answer your question on price - the County is allowed to charge a filing fee (usually the same fee as for filing a new Civil Law Suit) and once the case is filed, the DA will have to be served and generally, as long as the Statute of Limitations has run, they do not contest (BTW: Your lawyer will have to calculate the SOL, check verify things so there is more than just paperwork here). Once the Judge signs the order, it has to be served on every agency that is "Ordered" not to disclose - this would include the Clerk of the County in where you went to court, the District Clerk, the District Attorney, County attorney if one exist, the sheriff's office, the arresting agency and if there was a transfer the second law enforcement agency, the Department of Public Safety (State Troopers), TCIC, NCIC, Public Data, People Search (you get the idea, there are between 12-18 agencies to serve notice of the Order upon depending on your specific facts). Orders have to be served by the Clerk and the clerk can charge for this service (ironically, they can even charge you to serve themselves) - the fee where I practice is currently $15 per notice (it can and does vary). I generally estimate $500 in fees to the Court for filing, service and notice. Then there is the attorney's services to pay which varies by firm/attorney and jurisdiction and you can plan on paying for 4 or 5 hours of the Attorney's time at whatever rate he/she charges.
I know I did not give you a direct answer and I assure you I am not trying to be vague. The problem with what appears on it's face as a simple question is that there is no simple answer. The best I can do is tell you that if this happened in Collin County, my firm charges $750 plus cost to handle these cases. We do not know the exact fees charged by the clerk until we look at your specific facts and case (ergo why I estimate court cost at $500) and I am generally close.
If you are looking to get a Collin County case "non-disclosed" I would love to meet with you and get more information. You can also get more information on me and my firm at our website. I invite you to check us out.
Answer Applies to: Texas
Unless you were acquitted of the offense or case was not prosecuted after arrest, you cannot get the case expunged. If you pled guilty and were given deferred adjudication probation and successfully completed the deferred adjudication probation, then you can request the court to grant an Order of Non-Disclosure which essentially is an order to seal the records from the general public (some agencies will still have access to said records). My office generally charges $500.00 plus court costs for a misdemeanor case in Collin County, Texas.
Answer Applies to: Texas