If you are a SaaS vendor, you are probably selling your product by a monthly subscription fee.
If so, then you should offer something like 10% of all monthly subscription fees of the customer to the outside sales person who acquired the customer.
It has the pleasant effect of aligning their interests (making money by making the sale) with your interests (making recurring subscription fees). With a low up-front commission, sales people are more likely to pursue people who want a long term relationship with your company. Another pleasant effect is that customer acquisition and retention costs become very predictable.
The recurring commission also gives them an incentive to make sure that there is no attrition. For example, they can monitor usage and data growth of their sales, and if the numbers look bad, they can make a follow-up call to inquire why.
A flat commission for a SaaS product motivates sales people to make sales, and not necessarily to acquire customers. This could end up being expensive for you. Also flat commissions encourage sandbagging of sales to help sales people remain on the "safe" part of the sales leader board, and encourages sales people to go beyond quotas. It also becomes harder to predict customer acquisition and retention costs because you never know if the big commission you paid is for a short-term or a long-term customer.