Building a real bridge is very expensive. There are temporary bridges that can be even rented but again not cheap. Everything depends on the situation on the ground. A flat rail car can be as low as $10K but that's not installed or transported and depends on condition, but I point them out as likely more affordable solutions but they can only be used in certain situations, vary by length, also limited in width. Have to usually be installed by crane, but any structural bridge is a very costly endeavor unless its just homemade for a residence and car - and even then, has to be engineered, and then, what is the weight capacity in case a loaded truck needs to cross it? It all has to be engineered but the advantage is the flat rail car is already fully engineered with specific load tolerances so that cuts out a lot of the guesswork and can be installed faster - again depending on a situation. They work in the right situation.