In the best case, if someone has a roster spot and $50,000 he must be on a Major League roster for at least 90 days and active (not on the DL)the next season. In the lesser cases, if you are trapped in lower minors in one organization, someone can break that logjam and move you up a level or two for a small fee.
Some Rule 5 Draft info from Wikipedia:
Selection eligibilityPlayers are eligible for selection in the Rule 5 draft who are not on their major league organization's 40-man roster and:
– were signed at age 19 or older and have been in the organization for four years; or
– were signed at age 18 or younger and have been in the organization for five years.
If chosen in the Rule 5 draft, a player must be kept on the selecting team's 25-man major league roster for the entire season after the draft—he may not be optioned or designated to the minors. The selecting team may, at any time, waive the Rule 5 draftee. If a Rule 5 draftee clears waivers by not signing with a new MLB team, he must be offered back to the original team, effectively canceling the Rule 5 draft choice. Once a Rule 5 draftee spends an entire season on his new team's 25-man roster, his status reverts to normal and he may be optioned or designated for assignment.
To prevent excessive turnover in the minor league levels, each draftee costs $50,000. If the draftee does not stay on the selecting team's 25-man (major league) roster all season, the player must be offered back to his original team at half-price. Organizations may also draft players from AA or lower to play for their AAA affiliates (for $12,000) and may draft players from A teams or lower to play for their AA affiliates (for $4,000).
The Rule 5 draft has opened opportunities for teams to take other teams' top prospects who may not be ready for the major leagues. A prominent recent example is Johan Santana, who was chosen in the 1999 Rule 5 draft by the Florida Marlins when the Houston Astros declined to put him on their 40-man roster. The Marlins traded Santana to the Minnesota Twins in a pre-arranged deal for cash and Jared Camp who was taken in that same rule 5 draft by the Twins immediately before Florida chose Santana.
Notable Rule 5 Draft Picks: Roberto Clemente, Paul Blair,Jose Bautista, George Bell,Bobby Bonilla, Josh Hamilton, Scott Podsednik, Dan Uggla, Shane Victorino. All these became MLB All Stars.
Other Notable Players Drafted but returned to original team because drafting team did not open a spot on roster: Ivan Nova of Yankees, Cecil Cooper.