Lavatory

ABS trap is super cheap and attaches tightly and positively. It helps to have an extra long stubout for cutting back to length -- or a super short one onto which to attach a coupling sleeve. 7/16" lap at ABS joints.

No leaks so far (knock on wood)...

$53 faucet +
$150 integral sink and side splashes +
$100 marble cutting +
$35 connector hoses, valves and plumbing =
$338 subtotal in Powder room