Where you getting 2.5? It costs about that for just the first rounder.
http://www.spotrac.com/nfl/dallas-cowboys/cap/
When you calculate the top 51 salaries, none of the rookie salaries will actually count against the CAP during the summer because ALL of the base salaries will be the rookie minimum. On a 90 man roster, all of the rookie salaries won't count. So, the only CAP room you need to sign the rookies is the CAP room needed to cover the prorated 2018 portion of the rookie signing bonuses.
Based on the rookie salary scale, the Cowboys will need a little under $3,012,000 to cover the cost of the rookie signing bonuses - including all the signing bonuses for undrafted rookies, which is about $92,000 of the $3,012,000 they need. By cutting Orlando Scandrick, and making him a designated June 1st cut, his entire scheduled $3,000,000 2018 salary will come off the books on June 1st. That means the Cowboys only need a little under $12,000 in CAP space to sign all the rookies. The $480K apiece (total $4,800,000 for 10 draft picks) won't count against the CAP during the summer. Neither will the $480K salaries for the 15-20 undrafted players the Cowboys sign in the Spring. Only the top 51 salaries count. But ALL the bonus money allocated to 2018 counts - including bonus money for past players, and players who get cut during the pre-season.
Now, when it comes time to cut down to 53 players, the entire salaries of all the rookies WILL count. But by that time, the Cowboys will have cut the higher salaried players, and their numbers will be removed from the CAP, and many of those rookies will also be cut. At that point, the Cowboys also need about $1.2M to cover the Practice Squad salaries for 10 players, which are about $120K each. That is in addition to the salaries of the 53 players who make the team, plus any players put on injured reserve. The Cowboys like to keep about $2-3M in CAP space to make more moves during the season as players get injured.
So, that is why the Cowboys only need about $3M to cover the rookie contracts during the summer.