It's under the GPL, not the AGPL. Provided that you're using it as a web platform, why would this end up being a big deal? The client has your source anyways, right, in the form of client-side JS?
Also, have you talked to them? Like, actually written an email? Maybe it's not so bad.
If you haven't even figured out your revenue stream you probably are prematurely optimizing choice of license. Besides, you could still make use of it for prototyping and figuring out better your product requirements.
Meteor mixes server- and client-side code, and in general the two sides will interact so intimately that as I understand the GPL, simply separating the server-only code into its own files won't be enough to avoid the requirement to provide source code for it. The Meteor developers might have a different understanding of this, though.
Also, have you talked to them? Like, actually written an email? Maybe it's not so bad.
If you haven't even figured out your revenue stream you probably are prematurely optimizing choice of license. Besides, you could still make use of it for prototyping and figuring out better your product requirements.