What's Driving the Spears-Federline Custody Battle?

Sources connected with the custody battle between Britney Spears and Kevin Federline tell TMZ that at the heart of the tussle over the kids is nothin' but cold, hard cash.

K-Fed is going back to court asking for more custody of the kids. Federline just agreed to a 50/50 split, then turned around almost immediately and filed papers asking for 70/30.

Why the sudden change of heart?

It's puzzling that Kevin is now so alarmed, given that he's had zero contact with Spears for three months.

In fact, Britney Spears has been begging him to communicate because of parenting issues and he reportedly won't agree.

Which brings us to the legal paperwork Kevin Federline recently filed.

Sources say there are declarations from Kevin, both of his lawyers, Kevin's nanny and a nanny who interviewed with Britney in October, 2006, before the couple separated.

In the papers, Federline's lawyer also included various celebrity news articles about Britney. Legal experts say good luck getting that into evidence.

As we talked about earlier, Alli Sims, Britney's cousin, got served this weekend.

Kevin's lawyer wants to take her deposition. But here's what's telling - K-Fed's lawyer also wants to take Britney Spears' deposition, but a source close to Alli Sims says the deposition notice Brit received is all financial - the references are all about money, not the kids.

So does K-Fed want more custody of Sean Preston and Jayden James just so he can get more cash? A Federline source says it's all about the kids and not money. Kevin wants Britney under oath talking about whether she drinks and how she raises the kids.

Oh, and one last thing. FedEx's lawyer, Mark Vincent Kaplan, is asking that Britney pay his legal fees for the hearing and preparation. And that means six figures.

We assume "six figures" refers to the sum of money at stake, rather than the number of "figures" Britney has slept with since filing for divorce from Kevin.