I think people would be surprised how often 100% is offered. Still it would be considered very rare. It's all based on how many and which schools are bidding. These are players that are considered certain impact. College All American types. Three years and get drafted high.
However, many of these full ride types never set foot on campus. They get drafted very early out of HS and start a professional career. That is one of the major reasons why certain colleges look like they over recruit. it is also a reason why there is often money left to sign late bloomers. Every June, sometimes before if a player becomes a certain early draft pick and has expressed he wants to sign, a college he has committed to will be looking for a replacement and all of a sudden they have found the money to get one.
Think if you were in charge of a top college program. Many very good players will want to play at your school for nothing or 25%. Would you want the 6 very best players/.pitchers in the country and then use the other 5.7% to fill your roster. Or would you want to get the next best players and spread your money out? Which team is likely to be more successful? Of course nothing is guaranteed because there are other factors. Some programs are better at development than others, probably being the biggest factor.