The cheapest and quickest way to get to Varadero from Havana is by taking the shuttle Cubanacan Conectando. You can also take a taxi for 1 hour or Viazul bus for 3 hours.
The Shuttle service Cubanacan Conectando costs £7 – £11 (about $12) and takes 1h 50 m. Source: https://www.viajescubanacan.cu
You can also take the bus Viazul. The jurney from Havana to Varadero take about 3 hours and costs £7 – £11 (about $12) . Source: https://www.viazul.com/