It may or may not be a bargain, but they won't give it away-XFL inventory that is. WWF executives say ad agency estimates in Broadcasting & Cable's cover story on the XFL July 31 are offside. Those sources had said that a 30-second unit in the XFL on NBC next year would cost between $17,000 and $20,000. Sources at the WWF who will sell the XFL inventory weren't talking then. But now they say spots in the three national games on NBC, UPN and TNN will be sold as a package. They estimate the combined value of a spot that airs in the three games will average about $145,000. They expect to name a group of charter advertisers in the next week or two.