Since language is all about the communication between people, I'm a firm believer that you do not need to spend a lot of money to learn a new language. You just need to get out there and talk to people; however there are a few things worth spending money on, such as, a good dictionary and verb conjugation manual.
Although you can find a lot of very useful tools for free online, like online dictionaries, there's nothing that beats your very own hard copy dictionary. This is especially true, if you're a kinetic or physical learner. The hands-on approach helps to reinforce what you are visually learning, and it allows for you to make personal notes about things that are important to you.
Also, no matter how important the cultural aspect is in language learning, the base of all languages are vocabulary and grammar. If you want to be an effective communicator, then you need to make sure that you are saying what you think you're saying, and since Spanish relies so heavily on conjugation, making sure you have at least a few verb tenses down pat are extremely important.
You can never go wrong with the classics, many schools rely on the Webster's Spanish-English Dictionary for Students, Second Edition, and for good reason. It's accurate, easily available, and easily understood. It's always on my bookshelf.