Sorry, but you need to get your priorities in order:
I have designed a PCB with a 2.4GHz PCB antenna on it. I have no experience with antenna designing ( but have recently gained a little bit knowledge after reading a lot online).
1. You have no experience with design. This isn't a problem in the long run, but it's a cost. It could take weeks to years to become familiar with antenna and transmission line theory (depending on how familiar one needs to be). In the course of that, you'd learn the tools necessary to execute a design confidently: EM field solvers, microwave simulators, how to use a VNA and evaluate the finished device, etc.
Besides the project timeline cost, you'd incur at least as much labor in the process, or school expenses (for a more classical education). It's not a cheap process, however you slice it!
(To say nothing of regulatory testing.)
Which brings up your second concern:
1. I read a few places that the bottom layer of PCB should not have any components on it (for antenna performance purpose) and you should try to work on a four layer board. Where in that is not the case with me. Unfortunately four layer boards increase the cost of production and are also tough to design . How will my antenna perform in this condition, any rough guess after viewing the images?
2. If you are doing a few-off build, the cost of a
32-layer board is still far exceeded by the cost of designing and evaluating this properly!
If this is going into small production (~1000s/yr?), the cost of development is still greater than the PCB cost, no matter what you use.
If this is going into higher production (>10k/yr?), the FCC will hear about it sooner or later, and regulatory approval will be critical. Tack on another $10 or 20k of lab time.
(Note that, you technically
need regulatory approval if you're selling more than one, period. Most companies do regulatory approval for their standard products. You might
get away with selling thousands over many years, without approval. And I've seen this happen before. As with all things, it's a business decision: do you want to get stuck with C&D letters, frozen sales, or fines, at some point? Is that risk worthwhile?)
3. Use a pre-cooked module.
These are CHEAP. Like ten bucks cheap. Even less from China. Even the Chinese ones are FCC and CE approved. As long as you follow the usage for the module, you don't need to test your finished product as an intentional radiator.
You don't incur any project timeline impact (or, at most, a few days to shop for one, make the footprint, and pick up the basics on programming it). The BOM cost is marginal. You can use 2-layer boards without worry (no RF touches the board, aside from the antenna being along the edge; or, at worst, a pin is provided which goes to a coax connector for an approved antenna type, and all you need is a short trace, connector, and lots of vias and ground pour).
As you can see, there are many good reasons why modules are so popular!
Tim