Double your current proficiency bonus
The expertise feature says the following (emphasis mine)
At 1st level, choose two of your skill proficiencies, or one of your skill proficiencies and your proficiency with Thieves' Tools. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.
The text says your proficiency bonus is doubled for these skills. If does not say that you get your initial proficiency bonus as a bonus.
In other words if your proficiency bonus is +2 (total +4) your expertise skills get an extra +2, if your proficiency bonus is +3 those skills get an extra plus +3 (total +6).