I am a freelance translator, but my strategy on whether to submit a proposal or not may apply to writers, too. In general, I only bid on jobs that have a detailed description and are crafted in a professional way, specifying the scope and requirements for the job. It does also help to check a the client's average hourly rate - personally, I find that way more predictive than the actual budget (which may be a placeholder).
Last but not least, you can also check the work history of the client, and the freelancers he hired previously. Did he hire only based on the cost (i. e. freelancers with little to no work history who probably bid lower to build a reputation) or is he looking for experienced freelancers.
After a while, you will also develop some kind of a "gut feeling" about the job postings you read.
It is possible that a client is legitimately searching for a long-term partnership when posting a first "trial" job. But, if the client really wants to see freelancer's best work, it would be unusual for them to pay less for that single trial piece. It would make more sense for them to expect a discount when offering a large contract, rather than requesting the single trial work. So my advice is, don't lower your fee for the "test" piece. Establish yourself and your pricing from the very beginning of the relationship.
Note: I know it is very easy to be drawn to the siren's song of a long-term contract. I recently fell for a big promises low commitment client. I invested time negotiating and crafting my proposal based on the anticipated value of the ongoing relationship rather than the actual value of the contract before me. It is important to find a balance between taking risks in hopes of big payoffs and investing your time in projects that put money in the bank.
FWIW, I'm a generalist. I do tend to "specialize" in blogs that are meant to be used as a marketing device for the client but I don't focus on one area or topic.
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