Your husband is submitting his tax returns because he works for cash-in-hand and does not receive salary slips and he does not deposit his salary in the bank. He should go back far enough to establish that his financial status is sound. Based upon what you wrote, there is only one tax return available, so he should submit it. Understand that this is a weak position to be in for a UK visitor application. They don't like it when somebody works cash-in-hand because the applicant will think it's OK to do that here in the UK. And a single prior year's tax return is barely admissible as evidence; I would be surprised if the ECO even looked at it.
For your question about proportionality, an exact amount cannot be computed without seeing lots of personal information (which would not be appropriate here); there is no exact figure. You have asked if a trip that depletes 1/4 - 1/3 of your husband's savings is proportionate. I don't think it is, especially if he has no documented way to replenish it when he returns. But this is strictly a guess. The ECO will make a guess also, that's all anyone can do; but the ECO has the advantage of seeing all your stuff so his guess will be much more informed (and that's what he does for a living).
The rules are pretty clear that there is no minimum amount they need to see in order to apply successfully. But on the other hand, the applicant's circumstances need to make sense. Given that your husband has just received his first refusal (where they threw the book at him -- 'a' through 'e') and his financial evidence is decidedly weak and he is overextending himself financially by depleting his savings, you may wish to wait until things improve or to visit him in Ghana. I pointed out in an earlier answer that BHC Accra is the most captious and demanding issuing post on the planet. If he receives a second refusal then matters will be worse.
Related question: Re-applying for a UK visa after getting a refusal due to unexplained deposits