Free isn't free. There's still no “free lunch” in life, and this holds true for internet connectivity, even in coffee shops. What you pay for in a “cup o' joe” covers the Wi-Fi. The same premise applies to connectivity costs at event venues and also guides negotiations. Something (or someone) has to cover the cost of providing it.
Know your usage history. The group's history on F&B consumption works its way into negotiations with venues, and so should connectivity usage. Know the group's peak number of wireless devices in use, as well as its history of bandwidth usage which may be delivered in your post-con meeting. The more you ask and know, the better your chances are for getting a good deal.
Calculate net needs. Typically you'll have various groups with differing connectivity needs. Attendees, exhibitors, presenters and event management staff will have low to high data flow requirements. Understand these needs before you sign a venue contract. A good starting place is the APEX - Event Bandwidth Estimator.
Engage a trusted advisor. Legal issues surrounding meetings can be complex, and so can bandwidth issues. It may be appropriate to engage a tech advisor/consultant to determine a venue's capabilities as they relate to your group needs just as you would hire an attorney for contract matters.
Know who manages the connectivity. Some venues have tech specialists with lots of experience and know-how. Others may be run by the general manager's cousin's nephew. Look at who provides the internet service and manages the network. It may be beneficial to get an outside bid if you think the facility can't meet your needs or you think you're being overcharged.
Negotiate by tiered consumption. Tie rates for bandwidth to consumption – for instance, if your group is 200 people and they each carry two mobile devices, then they are medium users. Or, think in terms that allow for email management (low) to video streaming and a private network in the exhibit hall (high).
Sponsoring connectivity. Get sponsorships to defray your event’s Wi-Fi costs, when possible.