Button left the Brawn team, now owned by Mercedes, to join the 2008 champion Hamilton at McLaren earlier this week.
"I thought the challenge of going up against Lewis in his environment would be exciting," he told BBC Radio 5 live.
"I'm earning less than I would have at Brawn, so it's not for money - everyone involved knows that's the case." 
Aside from Raikkonen, Hamilton and Alonso, similarly ahead of Button on the list are Brawn-bound Williams star Nico Rosberg, Massa, Toyota veteran Jarno Trulli and Red Bull Racing pairing Sebastian Vettel and Mark Webber. Whilst four drivers on the grid – Kazuki Nakajima (Williams), Romain Grosjean (Renault) and Adrian Sutil and Vitantonio Liuzzi (both Force India F1) – went unpaid, the total salaries nonetheless equalled a mind-numbing $134.8 million, or to put it another way, an average of $6.4 million per driver.
“There may be a global recession going on,” remarked Tom Rubython, who compiled the list, “but being an F1 driver means you earned a fortune this year. These figures are just basic salaries, and don't include sponsorship and other income, which can often be three times that amount.”
Full list of F1 2009 driver salaries:
1.
Kimi Raikkonen $45m
2.
Lewis Hamilton $18m
3.
Fernando Alonso $15m
4.
Nico Rosberg $8.5m
5.
Felipe Massa $8m
6.
Jarno Trulli $6.5m
7.
Sebastian Vettel $6m
8.
Mark Webber $5.5m
9.
Jenson Button $5m
10.
Robert Kubica $4.5m
11.
Heikki Kovalainen $3.5m
12.
Nick Heidfeld $2.8m
13.
Timo Glock $2m
14.
Giancarlo Fisichella $1.5m
15.
Sébastien Buemi $1.5m
16.
Rubens Barrichello $1m
17.
Jaime Alguersuari $0.5m
18.
Vitantonio Liuzzi $Nil
19.
Adrian Sutil $Nil
20.
Romain Grosjean $Nil
21.
Kazuki Nakajima $Nil
What the F1 teams spent on drivers in 2009:
1.
Ferrari $53m
2.
McLaren-Mercedes $21.5m
3.
Renault $15m
4.
Red Bull Racing $11.5m
5.
Toyota $8.5
6.
Williams $8.5m
7.
BMW-Sauber $7.3m
8.
Brawn GP $6m
9.
Scuderia Toro Rosso $2m
10.
Force India $1.5m