A double din head unit upgrade in your base + sports is a great alternative to the $7K or so that OEM navigation costs in the Z. You can spend anywhere from $400-$1500 on an aftermarket head unit depending on whether you get an older model (still BNIB) or the latest and greatest. Installation should run a couple hundred depending.
I would find an audio specialist in your area rather than buy something that might not be what you want in the limited Best Buy selection. Check the audio shop's pricing if they purchase and install, and ask for a separate price for install only. Price the HU online (Crutchfield is a good start) and compare to see which route is best for you - it's easy to order it online and bring it into the shop. Do your research and know what HU you want ahead of time based on features and reviews.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |