Baseball America recently studied what became of unsigned draft picks in the 2000-2009 drafts. For those who have been redrafted, BA found the following averages:
Redraft round for  high school non-signers from rounds 1-10, 2000-09:
Mean = 12.6
Median = 7
Mode = 1
Redraft round for  college non-signers from rounds 1-10, 2000-09:
Mean = 7.7
Median = 5
Mode = 4
We would have to consider exactly where in the first ten rounds these players were initially drafted, but all in all the risk of a high end draftee playing 1-3 more years of amateur ball doesn't seem to be overwhelming. There is clearly more downside for high school players, as evidenced by the 12.6 mean redraft, but the upside is there as well, as more of those players were redrafted in the first round than any other round.
As for the Rangers...
Here are the Rangers on this list:
Purke is eligible to be redrafted in 2011 and will likely be a top five pick. Lemke is eligible in 2012. Barragar was never redrafted.
The Rangers have generally not been burned by their unsigned picks. Godwin and Baker were picked in the first few rounds, but Godwin totaled three MLB AB and Baker never reached AA. Maxwell is currently in the majors with Washington but at age 26 has yet to establish himself.
One pattern that is apparent is the increased number of unsigned players under Jon Daniels. Texas did sign all of their picks in the first ten rounds in 2010, but in 2007-09 the club failed to sign six players, matching the number from the first seven drafts of the decade. There were some extenuating circumstances in the Daniels drafts. The Rangers possessed five of the first 54 picks in 2007, making it difficult to sign all of the high upside picks. Nash and Gast joined in that 2007 draft by 11th and 12th rounders Anthony Renaudo and Drew Pomeranz, who were drafted 39th and 5th overall in 2010. The club battled deteriorating finances in 2009, with agreed upon bonus range for Purke being pulled off of the table due to a lack of available cash.