I have been recommending Twilio to my clients, though I am not sure on thier rates
Informatica SMS seemed like the best option. After discussions with Twilio, they seemed to cost more and offered very little support for Marketo users. Twilio does appear to have more SMS tools if you are making a serious investment in the capability.
Since you are not, Informatica may work best for you. Costs are based on the Long Code or Short Code (very exp) + per text costs. I doubt you can do this for $50/mo in total costs. It works with a webhook and you can get responses, but that requires additional work.
Also, you need people to opt in first in most countries before you send. And then you need a mechanism to get the opt out.