I believe you need to put yourself in Cleveland's shoes (as well as the Texans) when thinking about trading back. The deal has to work for both sides, otherwise it won't happen. If I was Cleveland, I might rather trade with the Rams than the Texans. At second spot, two of the top 3 QB's (Teddy, Bortles, Manziel) will still be there. The trade charts only show a difference of 800 pts between 4 and 2, rather than 1200 between the 4th spot and the 1st pick. So Cleveland stands to give up a lot less by moving to the 2nd spot.
The Rams would probably be a willing trade partner, since they would be picking 4th, and the top 3 teams (Texans, Cleve, Jax) figure to pick a QB, while the Rams are probably more interested in an OT (Mathews). That would leave Mathews still available to them at 4, and probably Clowney as well. So the Rams basically lose nothing they want and acquire picks.
So if the Texans want to trade back, I think they have to compete with the kind of offer the Rams would take. In other words, I don't think we can realistically expect Cleveland to offer full trade value for the pick. In fact, the Rams have so little to lose, they might offer them a good deal. Going by the trade charts, a fair deal between Cleveland and the Rams would be to swap 2 and 4, and for Cleveland to throw in their 2nd and 3rd round picks (35 and 71), worth 550 and 240 pts, or 790 pts...close enough to 800 to be fair. A little better deal for Cleveland, and one the Rams might go for, is to swap 2 and 4, and Cleveland gives the Rams their other first round pick at 26, worth 700 pts.
The Texans would have to compete with that. The question, therefore, is how little would you take in exchange for the top pick in order to move down?