Before you begin to look at what you can and should spend to renovate your bathroom, you first have to consider what is the "norm" for your neighborhood. Everyone likes to put their own tastes in their home decor, however if your renovation is driven by improving the value of your home, how much you invest needs to be rationalized by what is expected in the area. In other words don't over improve for the neighborhood, and certainly don't go over the likely costs that a home in your price range would be expected to invest.
Example: A home with an appraised value of $150,000.00, with comparables in the area ranging in the $150-200,000.00 appraisal range would not yield a return on their investment with a $100,000.00 renovated bathroom. Even if you have the most lavish bathroom on the block, potential buyers in that price range are probably not willing to pay the extra $100,000.00 you spent on your renovation for a bathroom. If all the homes in your immediate area have average bathrooms that have not been upgraded on average in the last ten years then you are wise to make upgrades that are modest. Modest but not necessarily builder grade boring. You can pick a splurge item in your bathroom and then adjust the budget around that.
Suggested splurges would be a jetted tub, upgraded vanity or vanity top, upgraded stone tiled flooring. Perhaps if your bath and budget allows, a separate standing shower. The main thing to remember is that the budget has to make investment sense even if you don't think you will ever sell. Many times people sell later in life after the loss of a loved one, or a medical emergency that was unforeseen and in those instances we can not always predict how or what our future circumstances may dictate. With that reality in mind, a renovation could make all the difference in what you get for your home in a cash out refinance or if you have to sell.
Your home always needs a great appraisal to refinance and also to sell even if you don't plan on it, life may force it.
Remember, kitchens and baths need to be regularly updated about every ten years as a good rule of thumb. If you are still sporting a retro seventies bath or kitchen, and have a small budget, try upgrading gradually. One project here, another there. Just don't make the upgrades extend over a five year period or you could end up back at square one.