Since your investissement is in bitcoin, forget about dollard when you do the math !
if you put 10btc you should get back 10 btc, or it's a fails, period.
Only if one COMPARES an investment in bitcoin with an investment in SDICE does btc value of an investment matters.
To evaluate an investment in SDICE
an sich, USD value matters as btc value has little meaning. Bitcoins are not useful yet as a unit of account because it is not stable. Sometimes btc is worth a lot, sometimes it's worth much less. Saying something was worth x btc and now it's only worth y btc, does not tell the whole picture if in the meantime btc tenfolded in value!
Until bitcoin has become a mainstream stable currency (another few decades...) it makes good sense to continue to evaluate the performance of your investments with the most stable unit of account: fiat!