It's almost certainly a US mission, or possibly a European one, as the US spends more on space than any nation. Depending on how you pay attention, NASA missions or spy satellites are the most likely candidates to be that mission, plus any hugely expensive ESA missions. So let's take a look at those categories. Note that I'm looking for missions that are the most expensive by categories, often not explained, but...
- Curiosity- \$2.5 billion
- No space telescopes of any note launched.
- Parker Solar Probe- \$1.5 billion
- Rosetta- 1.5 billion Euros
- Tranquility ISS module- \$400 million
- JUNO- \$1.1 billion
So for NASA, it seems the most expensive mission launched in the 2010s was Curiosity.
As for spy satellites, well, that's a bit trickier. Those spacecraft aren't known for certain what they cost, but there are estimates out there. Candidates include:
So I would guess that pretty much any KH-11 satellite, such as USAA-224, is the most expensive payload of the 2010s, with an estimated cost of $6.35 billion. Of course, all of the latter missions being classified makes it really difficult to estimate the price.
Lastly, The James Webb Space Telescope cost an estimated 10 billion dollars, and was launched in late 2021, making it the most expensive satellite of the decade so far.