It's a '73 to '75 model - none of the 440 El Tigres were rare, unless they had the mod motor and pipes on them or were the Z's - here's the production run for each year:
1973 440 - 5,425 produced
1974 440 - 4,000 produced
1975 440 - 3,520 produced
1975 440 Z - 1,776 produced
If it's a Z, it's worth $1,500 on up, depending on condition - a standard El Tigre isn't worth near as much, unless it's mint.
What's the serial and model #'s?