The question might be simple, Carrie, but the answer is not. As with most questions in business, the answer is, "It depends."
Let me say at the beginning, though, that when you say "independent sales reps,' I'm going to assume that these are NOT W-2 employees (who would probably also get a salary and travel expenses reimbursed), but reps who are truly independent and work either for themselves or for a separate rep firm. Unless the agreement with the company says otherwise, truly independent reps pay their own expenses.
There is no such thing as a 'standard' commission rate for 'independent sales reps.' Rates vary, first, by industry. But that's not the only determinant. Second, does the rep have 'pricing discretion' (i.e., the authority to cut price(s) to make sales)? If the rep has such discretion, the commission should be based on gross profit, not selling price. And if the rep drops the price, well, of course, the G.P. will drop. . .but so should the rep's commission rate. That makes the rep think twice about dropping the price--not only will s/he get fewer dollars, but those dollars will come at a lower rate, too.
Unless there's a compelling argument to the contrary--which, I'll stipulate, might exist, but also which I've never seen in 38 years as a business professional--I recommend basing the rate on gross profit. I do that because the sales rep is closer to the customer than the company, and the rep should be able to make the call to drop the price to get the sale.
However, there should be a G.P. rate 'floor,' below which the rep gets 0 commission. The closer the rate gets to the floor, the lower is the commission rate. You can stipulate in the WRITTEN AGREEMENT--which you simply MUST have--with the rep that any sales with a 0% commission rate will be reviewed at least annually and might be rewarded with a bonus that is discretionary to the person doing the review. That way, the possibility of a 0% commission does have the potential to get rewarded. . .and every dollar that is above 0% gross profit does contribute to progress towards break-even.
There's also the question of whether the rep is paid on sale or when the customer pays. If it's the latter, then the rep will also become a collection agent. Only the company whose wares the rep is peddling can decide if that's a good idea or not. I think in most cases that it's not, even if the rep gets paid only after the customer pays. It's a not-great idea because it changes the customer's perception of the rep from the helpful sales person to the not-very-fun role of debt collector/deadbeat chaser. I believe it's best to keep the rep out of the collections business. . .although I do believe it is perfectly appropriate for the rep to ask for credit references and EIN on the initial sale. Those are non-confrontational, whereas a collection effort does confront the customer.
There are pluses and minuses to having independent reps. The primary pluses are (1) a company doesn't have an ongoing pay/benefits obligation and (2) no subside of a learning curve - the rep knows the industry and the potential customers. But those pluses comes with some significant minuses: (1) the company cannot exercise much control over how the rep spends his/her time, and (2) it's possible for a company to realize little revenue because of that lack of control. It's human nature for those working on a commission basis to peddle those wares that generate the most money FOR THE REP, which might not be the wares that produce the most revenue and profit for the company. To be sure, there's also a potential hit to the rep's reputation if s/he fails with a product line. But presumably the rep thought about that before agreeing to take on the line and spend time negotiating that written agreement.
I hope this is helpful. If you'd like a telephone chat, I can arrange that.