Six months and $30,000 sounds about right or maybe even a little low for a US secret clearance. There's a reason they're estimated to be worth about $10,000 a year. Insanely expensive and time consuming to get.

(OP has mentioned in another thread that this was not a secret clearance, but very similar.)

Top Secret clearance invesigations cost about $60k back around 2005ish when I was in the military. They might be a bit more expensive now.

Finances are, in fact, one of the things that are scrutinized very carefully. Susceptibility to bribes is a major indicator of security risk.

Federal check for secret/top secret or a state check for someone with access to certain public safety stuff. It's painful enough that many police agencies find it easier and cheaper to train cops.

They validate your references and ask references for other references. Takes a lot of time.

I'd guess it's more of a security clearance thing where, amongst other things, they send people out to interview your family/friends/employers/etc.