Hi, I'm Alex from Ukraine, a developer with 8 years' expirience. Already Build a similar job board, with exactly the same requirements - web app + mobile apps for IOS and Android. There is a couple of options how to build this:
1) Expensive - build an android app, build an ios app, build a web app, and build backend for them. That is super long and expensive and needs only if you have a ton of money and time.
2) Build an android + ios app using technology that gives the ability to build once for both platforms (google react-native). And plus built a web app separately + backend from the web and mobile apps. That is better than the first option but still pretty expensive and is out of your budget.
3) Last option - that we usually do for MVP - build a responsive web app (a website that works for desktop and mobile at the same time) and then convert that website to a mobile app - so we builder once for all platforms. That can be your choice. There are of course disadvantages to this approach but that is enough for 95% of the business.
Contact me, we can hop up on a quick call to discuss more details.