i want know is how much it would take to cost to fix this and how long would he have to go back to mexico for

If you are a US citizen and your boyfriend came legally to the country, you would be eligible for adjustment of status to permanent residence if you married. In such case, he would not have to leave the country. If you are a US citizen and your boyfriend entered the country illegally or under another status under which he is not adjustable in the States, he would have to leave the US to obtain an immigrant visa if you married and sponsored him. Under the new I-601A regulation, he would be able to apply for a waiver of the 3 or 10 year bar incurred by his illegal stay while in the US and before he decides to travel outside the country to attend an immigrant visa at an American consulate or embassy in his homeland. USCIS will begin to accept such applications on March 4, 2012. If the waiver is approved, the projected time outside the United States would be roughly a month if all goes well. An immigrant visa interview at that point should be fairly routine as the most difficult part of his immigration should be overcome by the approval of the I-601A waiver. On the other hand, if you are not a US citizen or do not plan to marry, your question does not give enough detail to speculate on other ways that your boyfriend might immigrate. As to your other question of how much it would cost to fix his situation, every attorney has his or her own rate. It would also depend upon which situation you and your boyfriend would present.  

Assuming your boyfriend entered without inspection and has acrued unlawful presence in the US, a waiver of the unlawful presence ground of inadmissibility is available if he can show hardship to a US citizen spouse (you, if you marry him) or child.  Starting March 4th, you can apply for a preliminary waiver while still in the US, eliminating the risk of a lengthy separation while the waiver is adjudicated at the consulate.