This is certainly in my domain of expertise having spent the last 10 years in the customer feedback/survey technologies and market research space.
The first question you need to answer is whether you need to conduct face-to-face (F2F) interviews or can it be done by some other means (web, phone-CATI). If you're issue is doing it on the cheap you can eliminate telephone surveys and possibly F2F unless you're willing to do it yourself. I would not suggest that. It's not as simple at it appears and to conduct 100 interviews you could be looking at many days of field research depending on the length of the interview.
If you want to gather your data on-line, then there are a myriad of affordable subscription-based DIY tools on the market. You named one in SurveyMonkey but there are others (QuestionPro, KeySurvey...). Just do a quick search and you'll find a ton. For the most part, these tools have easy to use authoring tools, survey admin features (invites & reminders, panel management) and reasonably decent, yet basic reporting. Some tools have built-in Conjoint Analysis tools. This is a type of analysis that is used to analyze consumer preferences. It's a heavy concept to get your head around if you're not a research analyst, so depending on the type of analysis you want to conduct, you may need the help of a qualified statistician. Lots of freelancers around. Just Google.
If you go at it yourself with a DIY web survey, the next issue you will need to confront is where is your sample file coming from. Do you have a list of the target audience? Keep in mind that response rates for these types of surveys are extremely low so to get a 100 responses, your initial sample would need to be at least 1500-2000. You will likely need to factor in an incentive to get any responses. Which brings me to my last point: is it better to go to a panel provider. You can do this but it's not free unfortunately.