As you have been told doing the upgrade is extremely easy, it takes about 10 minutes. Apple even has the instructions in the owners manual...you did read the manual didn't you? 🙂 You can get a
500 GB replacement for less than $100 and be back up and running very shortly. If you have a good backup that even makes it easier. The site I've provided the link to specializes in Macs and even provides free how to videos.
If you do choose to have Apple do it, then you will need to contact Apple for a quote.
Regards,
Roger