There is no industry-wide standard, however different brokerages may have set commision ranges within their company that they can offer and this may limit what you can negotiate.
if you have to use a broker, pick a good one that knows the area well and ask them to not co-broke it, and negotiate 3-4%
Other than the possibility that you may be able to negotiate a lower commission, I am not sure what the benefit of working with someone that does not co-broke would be. Your home would be marketed to a much smaller number of people which may result in a lower selling price or having your home on the market longer than is necessary.
If your property is desirable, then I don't see the point of co-broking. If it is
really desirable, then of course I wouldn't use a broker at all.
You can negotiate a 3% commission no-broking with a broker who knows he can move the unit, much easier than you can negotiate 1.5-2% commission per broker if the deal is co-broke.
A good broker will have a list of clients looking for the unit you are selling, a well trafficked agency website, place an ad in the Times, etc.
The benefit of saving 1% or more on a 400k sale is 4k per 1%. Not much, but if you know you are priced right, then it's money in your pocket.