The problem with this approach is that when you launch a website and then want to do some seo for it, you cannot afford to wait for discounts.You can be looking around BHW Marketplace, there is always some discounts, even up to 75%+
as the user above said, especial on some big holidays.
You can negotiate maybe with some service that is not monthly packages, depend on what you want from the package,
then find people who are doing that stuff, a few of them and ask them for the price, and if you wanna work with them in the long term,
you can "kill"the price for a few bucks.
You don't negotiate better deals. You push it.
Offer a budget and use this phrase: "I only have $xxx available, sorry. Do you think we can use that budget?"
In 7 out of 10 cases, it's going to work.
Now, the other problem, and a more serious one is, do you REALLY want to negotiate the SEO package? I know the answer. Do you?