Consider the following:
A 1st level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 10th level(and half way to 11th), a gain of 10.5 levels.
A 5th level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 11th level(and half way to 12th), a gain of 6.5 levels.
A 10th level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 14th level, a gain of 4 levels.
A 15th level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 18th level, a gain of 3 levels.
A 20th level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 22nd level, a gain of 2 levels.
A 25th level character draws the card and gains 50,000xp. Thats enough xp to raise the character to 27th level, a gain of 2 levels.
The lower the character level the larger the level gap will be between that character and the rest of the party, thus making balanced adventures harder for the DM. It will also cause the higher level character to steal some of the other characters thunder.
I think a better result from the card would be to grant a permanent +20% bonus to all xp earned. This would allow the character stay about a level higher than the rest of the party(if xp gain is equal to rest of the party) and be a benefit to all characters regardless of level.