Riolu (Baby Lucario) is only available via hatching 7km and 10km Eggs. It has a 0.7% chance of appearing (~7/1000). On average, each player will need to hatch 143 eggs to get a Riolu. With 3 uses per incubator, that’s a total of 48 incubators used. That comes out to a total of 9,600 COINS for Super Incubators, or 7,200 with normal incubators. This is equivalent to $66.20 USD and around $50.00 USD respectively, if buying coins from the shop. If you wanted to do this with only coins earned via gym defense, it would take at least 192 DAYS due to the 50 coin limit. Given that Riolu/Lucario were only released in October 2018, under 168 days ago, it actually hasn’t been possible to earn that many coins yet. The average free player with only the default incubator would have to walk just over 1,000km (621 miles) with only 7k eggs to get a Riolu.
Please keep in mind that I am discussing averages. Some players may get multiple Riolus in a row, some may get none. These statistics will be accurate when looking at a large sample of the playerbase.