The WNBA has a union. That union should do more for its card carrying dues-paying members, if there's a reason to act. That's my first thought on the matter.
Secondly, there are two popular models to pay athletes in a league where it's essentially a bunch of operating franchises (teams) owned by independent groups, who come together as a collective under a unifying banner (and try to walk the tightrope of collusion and profit).
The salary cap model (see: NHL, NFL, etc...): union barters for a certain percentage of league-wide revenue projections (the budget) and teams are forced to commit at least a certain percentage of that salary cap value to pay all players on their team, with no allowances to pay more. Upside: parity amongst teams and competitiveness. Downside: if unions are weak, downward pressures on payouts as a percent of league revenues (or off-shoot revenues from gambling that are hard to pin down) can leave players underpaid.
The luxury tax type of model (see: NBA, MLB, etc...): there's a soft cap, with teams permitted to pay more for players (the carrot: big city/big market attracts big players who play for big money to attract big TV and big gambling), but at the expense of having to return money to other franchises in the form of a luxury tax. Upside: small market teams can take the funds to try to remain competitive and invest in alternative ways to win/earn profit that flank the large franchises guerilla-warfare-style. Downside: incentive to just stop caring about fielding a competitive team, take financial losses, earn the luxury tax, and use the team as a write-off on capital gains with other companies in your portfolio. (the townsfolk get unhappy, etc...)
If WNBA players are paid an unequal percentage of league revenues versus men, then I'd say there's a case there to argue for the union.
But if we're arguing dollar for dollar, then WNBA league-wide revenues would have to approach NBA revenues. That's tough one; the NBA has been around far longer, has enjoyed long-standing TV contracts that perpetuate to some extent, and has a richer history in gambling than the WNBA. Some of that simply takes time, if the product is compelling enough to draw the attention of the mouth breathing masses. Who's to say it will or won't? There's never been any accounting for taste in product marketing, last time I checked (unless you ask marketers...and they'll tell you they know)! LOL!