If you asked me what a good but inexpensive restaurant I would recommend on Nevsky Prospekt in St. Petersburg, I would say Sirtaki.
The Greek tavern "Sirtaki" is located on Nevsky Prospect, at a distance of 200 meters from the Vosstaniya Square.
Sirtaki is probably not the best restaurant in St. Petersburg. I just can't compare, but your can find information about the best restaurants i2019 here. (There is, for example, the Palkin restaurant not far from this place.)
This is not the cheapest place to eat in Petersburg.
But I (and some of my friends) liked it.
According to information from the website of the restaurant: Every day in the tavern "Sirtaki" live music from 19.00 to 23.00! On weekdays, the restaurant also offers a business lunch.
Address: St. Petersburg, pr. Nevsky, 102.
The location of the restaurant is marked on the city map.
Opening hours (2020) from 08:00 to 23:00.