Cracking open the gearbox is generally a ~$500 minimum proposition, whether it is to replace a bearing or change a gear cluster (though gear cluster parts add alot more $$ to the bill than a bearing).
As indicated, you could change the stock final drive gearing from 3.56:1 or 3.44:1 to either a 3.36:1 or 3.2:1, but your speedometer will be off by a percentage corresponding to the ratio (e.g. going from 3.44 to 3.2 will net a 3.44/3.2 = +7.5% error in your odometer and speedometer readings, if you don't change them to match. Getting a replacement one (and possibly having to repair it) will then sometimes eat up most of the difference in savings from going the gearbox route, but at least the final drive/speedo change is something you can do yourself, versus pulling your transmission out and sending it to a specialist, like Anton Largiader, Tom Cutter, etc.
On the other hand, having the taller overdrive 5th gear in the transmission will still preserve your acceleration capability, which will be reduced across the board if you swap out to a taller final drive. Or, if you are changing to a 3rd part speedometer with front wheel sensor anyway, then the BMW part speedo-swap is a non issue and you can use whatever final drive you like.
Lastly, don't be put off/frightened by the 5000 RPM on the speedo when travelling on the interstate with the stock gearing - these engines are actually quite happy spinning like that, and you can cause alot more damage to it if you lug it around town in top gear at 2200 RPM..
